This lets us run a command like:
$ python manage.py councillors -t example -v

Which is useful when trying out commands that run multiple scrapers.

This pulls what was in the councillor command into the
PerCouncilCommandBase class. This is so that it can be reused.

Along the way I've:
- dropped reporting on skipped/missing/etc councils with the progress
bars - these can come back, but weren't working as intended.
- Made the command runner explicitly call `execute` rather than it
appear in the __init__ method.
- Added a 'pretty' flag to manage what output is shown.