#208 Add Lexer support for Asciidoc markup format files

[Anonymous]

Originally created by: shane.cl...@gmail.com

The Asciidoc text document format(see http://www.methods.co.nz/asciidoc/index.html)
is a very powerful markup language for creating books, docs etc.
(eg this book <http://git-scm.com/book/en/v2> is written using asciidoc,
the source is at <https://github.com/progit/progit2>)

Having built-in syntax highlighting support in bowpad would be awesome !