Expand 'license' command file filtering.

Currently the 'license' command only supports two file filter scenarios 'all files' or 'diff with HEAD' in the 'sources' mode.
This works unless you forget to apply changes after files where modified.

Additional behavior like:
- [ ] Only file with missing headers
- [ ] Diff against ref/branch/tag
- [ ] Override file extensions

Would help a lot in various scenarios.
