Make Github style relative links work with root Jekyll pages

[stevemoser]

Github added relative links to markup files that take the following form.
[Page in the same directory](page-in-the-same-directory.md)

Relative links are great because per Github:

This also means your documentation can now easily stand on its own, without always pointing to GitHub.

However Jekyll doesn't convert 'page-in-the-same-directory.md' to 'page-in-the-same-directory.html'.

Since I can't change Github I have to change Jekyll.

[stevemoser]

These fix #1. Next Github style relative links should work with _posts Jekyll pages: #2. Then with non-root Jekyll pages: #3.
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