Add sub-commands to mopidy and switch mopidy-scan over.

- Switches to sub-commands for serving under ``mopidy run`` (argparse is all or nothing with respect to sub-commands)
- Converts ``--show-deps`` and ``--show-config`` to ``mopidy deps`` and ``mopidy config``
- Cleans up bootstrapping of logging
- Adds hooks for extension to install commands
- Converts ``mopidy-scan`` to ``mopidy scan``

Note that it's intentional that the commands ``deps`` and ``config`` are run earlier before certain config validation steps etc have been run and do not use the generic mechanism setup for extensions.

Things missing before merge:
- [x] Update docs
- [x] Update changelog
- [ ] ~~Consider porting ``mopidy-convert-config`` to ``mopidy-config``~~
- [x] Update extension writing docs