Add a config for isort (https://github.com/timothycrosley/isort)
Run isort -rc . and make import style consistent across files
Add a note about `isort` in CONTRIBUTING.md
When file has no metadata field indicating the track number, Picard
is trying to guess it from filename, but the simple regex wasn't very efficient.
tracknum_from_filename() does a much better job.
A test case was added.