Add a makefile to automate tests

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
Make the support file depend on both input and fixture

So when a test input file is modified, a simple `make` will run
the test again because it would have noticed the input is newer
than the destination.
Sort tests so final `support.yml` is deterministic
Merge pull request #20 from valeriangalliat/makefile

Makefile