Useful if you're creating a section only to access it in the index but
do not want a section page for it

Closes #14, #43

Using a greedy regex could lead to unintended consequences, like the
inability to syntax highlight a unified diff. This resolves issues like
that by using .*? instead of .* so that the closing +++ is matched as
soon as possible.

Parse _index.md files as sections and render them