Interface for transmission's labels feature

[rsekman]

Since version 3.00 transmission now supports labels for torrents, e.g., music, movies, iso. stig should provide an interface to this feature, meaning

• torrent lists should have a column for labels
• torrent details should display labels
• it should be possible to filter torrents by labels
• there should be a command to manipulate labels
  • set a torrent's labels to a new list
  • add any number of labels
  • remove any number of labels
  • clear all labels
• add should take an argument to label torrents as they are added

[rndusr]

Thank you. Looks great, apart from some style issues. Running flake8 should point those out.

[rsekman]

Thanks! I think I got all of them fixed.

Thank you also for pointing me to flake8. This will help me lint the rest of my contributions. Previously I was sort of guessing the style.

[rndusr]

I'm also using isort for keeping imports predictably ordered. Also, I've decided I'm not going to put more work into stig's current code. It badly needs to be split into multiple projects, and the current design doesn't really allow for supporting more clients, at least not as easily as it should be. So, even though I generally welcome PRs, it's a waste of everyone's time at the moment. But I'll pull your existing PRs and probably maybe make another release with them.