Add failing test for new label parsing functionality
Add logic to parse PR description/comments for labels

With this commit, if any line starts with "--label ", the following content
will be treated as label and used for exclusion/inclusion.
Add integration test for label added to the PR description
Add extra spaces to the exclude DEBUG statements to match PR __repr__
Allow PR exclusion using `--exclude` and update integration tests
Return list of all excluded PRs in the merge command info log

- run_filter() now returns a reason together with the filter status
- unit tests have been fixed accordingly
- status-excluded PRs are now included as part of the excluded PRs section
Return empty string in merge_info() is candidate_pulls is empty
run_filter() should only log and return the sets intersection
Add whitelisting option in PullRequest to retrieve comments

Add unit test with a whitelisting function checking organization members
Turn whitelisting on when parsing comments for --exclude labels

- Check for exclude comments ahead of  PR filters and fail-fast if found
- Mark comment-excluded PRs separately
- Reuse the default filter mode for whitelisting PR comments
- Fix documentation of default mode in the command help
Merge pull request #125 from sbesson/125_label_comments

Teach `scc merge` to parse comments for labels