add include-excerpt feature to 'goal markdown'

Extend markdown syntax so that page authors can "include" a snippet from a real source file. Inspired by the way the Pants docs use Spinx's literalinclude http://sphinx-doc.org/markup/code.html#directive-literalinclude .

Added docs for it in src/python/pants/docs/page.rst

Exercise it in examples/src/java/com/pants/examples/hello/main/README.md

Testing Done:
Added test_markdown_to_html.py with a bunch of tests for the "excerpt a few lines" functionality, which has corner cases.

https://travis-ci.org/pantsbuild/pants/builds/33925158

Reviewed at https://rbcommons.com/s/twitter/r/949/