Add ability to reference link to other components in docs.

To link to the "typography" component on the "css.html" page from
another component's docs, you can now do
`[link to typography][typography]`
instead of
`[link to typography](/css.html#typography)'.
Add docs for component reference links
Move reference link logic to MarkdownRenderer

Signed-off-by: Geoff Pleiss <gpleiss@pivotal.io>
Merge pull request #171 from gpleiss/reference-links

Add support for reference links to other components