Add Sass support

This overhauls everything and adds Sass support to the styles.

- `.scss` files are stored in `_scss`.
- `css/styles.scss` compiles into `_site/css/styles.css`.
- This replaces the Liquid includes I was previously using for the syntax highlighting.
- Reduces two CSS calls—one for `poole.css` and one for the syntax—into a single file.
- Breaks styles up into individual `.scss` files.

This PR represents the bulk of the work going into v2.