Add function to insert mtime in stylesheet filenames

Alas, this is not as useful as I had hoped, because mtime changes on every
load. Thus, you can only save/reload a file once before the filename no
longer matches what's on disk.

I don't know how that wasn't obvious to me, but it wasn't.

Duh.
Move README -> README.md

since it is written with Markdown syntax, or at least something close to it.
Generate README.md from comments in package

Using md-readme.el.

It looks promising. I should really learn to use Cask then auto-generate
the readme as needed from a pre-commit hook.