Add support for .sls YAML files (SaltStack states)

This PR adds supports for SaltStack states files (with a `.sls` extension) as asked in #960.

I tested on [saltstack/salt-states](https://github.com/saltstack/salt-states) and [coffeestats/coffeestats-django](https://github.com/coffeestats/coffeestats-django). The detection isn't perfect but it's better than before (those files were detected as Scheme).
@arfon I guess we'll have to wait for the test system to merge this :-)

EDIT: Also fixes the issue with this repo: [uggedal/states](https://github.com/uggedal/states).