Allow individual rules to be negated.
Add alembic revision for that.
Provide an API to modify rule-negation.
Fix __repr__ logic.

As pointed out here:
https://github.com/fedora-infra/fmn.lib/pull/12/files#r16629573
Merge pull request #12 from fedora-infra/feature/rule-negation

Allow individual rules to be negated.