Uses SCSS instead of plain CSS

Closes https://github.com/realm/jazzy/issues/83

Also migrates to `Pathname` instead of a spaghetti of `File` methods
> Since we now print the relative path, I think it'd make more sense to use "in" instead of "at".

:+1:

> I'm also not sure wrapping the path in backticks is a good idea since it doesn't affect rendering at all.

It makes the path command-clickable in iTerm :)