Use yaml configuration files instead of current custom specification

[heuristicus]

The custom specification could probably be replicated in yaml without issue and would mean that the sheriff_config file would no longer be necessary. Perhaps there isn't much to be gained from this, but a custom parser and file format is not ideal.

[mcamurri]

I agree that would be nice, but since the internal parser works fine I would keep it that way.

Migrating to yaml would have the only benefit for new users, who do not have to learn a custom syntax for a config file. It is something we might want to consider in the future, but not now.