Restructure config files

I think that a default config section title of `$appname` (as in the `{afk, window, spotify}` watchers currently) is a bad idea.

- [ ] Change to a proper default section.

We should also handle testing settings using configparsers override functionality (override some defaults when run in testing mode), not as it's own defaults.

- [ ] Testing modes should be an override of the default, not a different default.

I also want to add comments to the config files to make them easier to edit.

- [ ] [Add comments to the config files](https://stackoverflow.com/questions/6620637/writing-comments-to-files-with-configparser), making them friendlier to edit.

It would be nice to have some kind of argparse + configparser settings union to simplify overriding parameters using the commandline.

- [ ] (extra) Nice way to use argparse output as overrides.