Propagate new check-commit-status options to find_candidates()

- Add log information when parsing filters
- Extend exclusion condition depending on the filter and status
- Falls back to reading the head repo status if no status on the base repo
- Improve the status-excluded PRs digest by exposing the read status
Closes #105
Expose ref to create_status() function and set to "base" by default
Add integration tests for the new --check-commit-status options