allow chunk output to be re-evaluated

[schrimpf]

This pull request adds a chunk option to be re-evaluated as code.

It partially addresses #89 by allowing an external file to be included as

    ```julia; output_as_code=true
    print(read("filename.jl",String))
    ```

When used in combination with Revise.jl and CodeTracking.jl, it also allows including the text of function definitions from modules in the Weaved document. For example, weaving the following document

    ```julia; output_as_code=true; output_eval=false                                                                                                                             
    using Revise, CodeTracking, InteractiveUtils                                                                                                                                 
    using Random                                                                                                                                                                 
    print(definition(@which rand(2)))                                                                                                                                            
    ```

produces

begin
    #= /build/julia/src/julia-1.2.0/usr/share/julia/stdlib/v1.2/Random/src/Random.jl:254 =#
    rand(dims::Integer...) = begin
            #= /build/julia/src/julia-1.2.0/usr/share/julia/stdlib/v1.2/Random/src/Random.jl:254 =#
            rand(Float64, Dims(dims))
        end
end

You can get nicer output by stripping comments with e.g.

'''julia; output_as_code=true; output_eval=false                                                                                                                             
using Revise, CodeTracking, InteractiveUtils                                                                                                                                 
using Random  
                                                                                                                                                               
function definition_text(method::Method)                                                                                                                                     
  function reformat(txt)                                                                                                                                                     
    # remove begin...end block and its indentation                                                                                                                           
    txt = replace(txt,r"^begin\n"=>"\n")                                                                                                                                     
    txt = replace(txt,r"end$"=>"")                                                                                                                                           
    txt = replace(txt,r"\n    " => "\n")                                                                                                                                     
    # strip line number comments                                                                                                                                             
    txt = replace(txt,r"(#= .+ =#)( |\n)" => s"\2")                                                                                                                          
    txt = replace(txt,r"\n\s+\n"=>"\n")                                                                                                                                      
    # change indentation from 4 to 2                                                                                                                                         
    maxdepth = 10                                                                                                                                                            
    for d in 1:maxdepth                                                                                                                                                      
      r = Regex("(\\n) {$(4*d)}(\\S)")                                                                                                                                       
      s = SubstitutionString("\\1"*prod(fill(" ",2*d))*"\\2")                                                                                                                
      txt = replace(txt,Pair(r,s))                                                                                                                                           
    end                                                                                                                                                                      
    txt                                                                                                                                                                      
  end                                                                                                                                                                        
  txt = string(definition(method))                                                                                                                                           
  txt = reformat(txt)                                                                                                                                                        
end                                                                                                                                                                          
                                                                                                                                                                             
using Random                                                                                                                                                                 
print(definition_text(@which rand(2)))                                                                                                                                       
''' 

which creates output with

rand(dims::Integer...) = begin
    rand(Float64, Dims(dims))
  end