Modify the default exclude parameters for all merge commands

- Set ['exclude', 'breaking'] as the default exclude labels/comments
- Teach find_candidates() to parse comments for any exclude label
- Allow find_candidates() to include PR using either PR attributes or include
  comments (for any include label)
- Fix filter unit tests
Add test case for PR inclusion via comments
Merge pull request #138 from sbesson/exclude_defaults

Modify the default exclude parameters for all merge commands