Add new checks

[ekadlecova]

Checks should be executed in this order:

• [ ] Check comments for patterns
  • [ ] matching existing files
  • [ ] avoid duplicates with existing patterns
  • [ ] ask to delete commented patterns that don't match existing files
• [ ] groups without comments
  • [ ] add comment
  • [ ] merge with other group
• [x] missing references
• [x] useless patterns
  • [ ] avoid duplicating patterns when suggesting new ones #7
• [ ] detect duplicate patterns
• [ ] move groups with same owner to subgroups of a "domain" group - ask for title

It should be possible to configure which checks to run (git config?)