Show warning when declaring non-existent rules.

[devminded]

To improve user experience a warning should be shown when trying to apply a rule that does not exist.

As seen in issue #106 I made the mistake of declaring "all-dependencies" instead of "all-dependency" in the rules block. Nothing happened and after a lot of trial and error I reported a bug. Just to figure it out an hour later.

Example:

WARN: Unknown rule 'all-dependencies'. Available rules are: all-dependency, foo, bar.
Did you mean 'all-dependency'?