Add hamdown support to tilt gem so it can be used as an engine in Rails and static site generators

[inem]

[inem]

Examples to start with:

• https://github.com/inem/tilt/blob/master/lib/tilt/builder.rb
• https://github.com/rtomayko/tilt/blob/master/lib/tilt/plain.rb
• https://github.com/rtomayko/tilt/blob/master/lib/tilt/rdoc.rb
• https://github.com/inem/tilt/blob/master/lib/tilt/asciidoc.rb

[inem]

https://github.com/rtomayko/tilt/pull/340