Support RNA sequences

[fedarko]

Probably the easiest solution is adding a boolean flag rna when creating a DotPlotMatrix (defaulting to False). If this is True, then the code should just work as normal but replace T with U. If either of the input sequences contain a T then this should cause an error, same as how (currently) if either of the input sequences contain a U then this causes an error.

This is similar to #12 / #13.

Edit: actually, it would be even better to just add a flag called seq_type or something that can be one of {dna, rna, protein} or something. It should default to dna and result in the current validation behavior. We can just save this as a property of the matrix object -- will make the code easy to interpret.