Fix and improve literal mode.

Fix and improve literal mode.

As per [request](https://github.com/jashkenas/coffee-script/pull/2838#issuecomment-15020652) (Sorry for the wait):
  * Require a blank line as delimiter between text and code.
  * Don't ignore indentation with mixed whitespace.
  
Address issue with #190:  
  * Treat blank lines as code or documentation, depending on preceding
    lines. This avoids starting a section for each blank line in
    documentation (which breaks markdown processing for lists).
  * Remove an unused variable.
  
It would also be possible to combine the two loops (over `lines`), avoiding adding and then removing comments. Not quite sure if this fits your style though.