Improve literal mode.

[marchaefner]

• Expect a blank line as delimiter between text and code (Fix parsing of indented markdown features by requiring line-break delimiter #2821).
• Don't change indentation of code. It is not necessary and leads to
  erroneous locationData. (Literate mode: incorrect column in locationData / source maps. #2835)
• Don't modify blank lines and reverse the change in the lexer.
• Don't ignore indentation with mixed whitespace.

There are still open problems:

• Some markdown processors will need an indentation of 8 space in/after a list to indicate code (nested lists require 4 more spaces for each level). So it's either markdown-parsing or some restriction on support.

• Multiline tokens can consume the non-code part of the document. E.g.

  some text
  
      str = """
  
  this line will get a # and be a string
  
      """
  

[marchaefner]

@jashkenas: This is actually meant as a more condensed, (hopefully) readable and location-preserving version of the code in #2821. (I personally find the second version a bit hard to comprehend, for the rather simple thing is does).

[jashkenas]

Lovely, thanks a million.

For super deluxe extra bonus points, consider submitting a version of this same patch to Docco as well ... and get your name on the contributors list.