Enhance pygments gherkin lexer to recognize markdown

[mhfrantz]

Groc uses pygments to parse our Gherkin .feature files and render them as .html. Pygments already supports both Gherkin and Markdown, but does not yet support Markdown inside Gherkin. It may be easy to implement.

Pygments lexer documentation: http://pygments.org/docs/lexerdevelopment/

Multiple lexers: http://pygments.org/docs/lexerdevelopment/#using-multiple-lexers
Gives an example of using the Javascript lexer from within the HTML lexer.