Add route and render

New functions take advantage of query library and make it much simpler
to process files by path.

Update lettersmith-markdown to take advantage of render.
This also means lettersmith-markdown only looks for **.md files and
ignores .mdown and .markdown now. Seems like an ok compromise for
simpler code, since .md is the most common.

Though this wasn't intended to be an optimization, I'm getting much
faster build times (200% to 400% faster -- down from 8s when processing
5000-odd files). I won't question it.