Officially unsupport multi-line attribute blocks.

[AnalyzePlatypus]

Closes #74.

HAML-js does not support multiline attribute blocks.
This PR clarifies this fact in several places.

• The error Malformed attribute block now reads Multi-line attribute blocks are unsupported in HAML-js
• Adds a new section, Differences from Ruby HAML to the Readme, and notes that:

Multi-line attribute blocks are not supported.

[Sinetheta]

I think that clears things up nicely. Spreading different attributes across multiple lines is handy for readability, but certainly not required. As long as users know they're not crazy, they need to adjust their markup.

[mebibou]

Having lines of 500chars because this library does not support multi-lines is not what I call readability, it's common sense. Has anyone ever wrote a function in js as a single line of 500chars? nonsense