Makefile

For each subdirectory in `tests`, minify `*.css` to `*.min.css`, compile `input.scss` with each supported Sass compilers using SassMeister web interface, then compare against the "expected" or "unexpected" fixture to compute the results in `support.yml`.

The `support.yml` of each individual test is then concatenated in a global `_data/support.yml`.

- Fixes #12