Add Haml 'support'

- If Haml is available and the user specifies 'haml_example' in the code
  block, convert haml to html for display purposes but render a haml
  example.
Support syntax highlighting for Haml.

- Was necessary to 'hint' the lexer for pygments.
Merge pull request #46 from pivotal-cf/master

Add Haml 'support'