Support GHM relative linking

Github is capable of relative links in READMEs. We support this style
of linking for generated documentation as well.

We post-process the markdown for relative links to other markdown,
adding the necessary ‘.html’.

A unit test has been modified to check for correct behavior.

[1] https://help.github.com/articles/relative-links-in-readmes
Merge branch 'feat-ghm-relative-links'

Conflicts:
	spec/lib/environment_spec.coffee