CLI subcommand refactor

Still in progress, this branch brings:
- CLI is now based on subcommands
- Database is locked only when executing tasks (you can now run series reports etc. while another instance is running)
- Old lockfiles are ignored if the PID that created them is no longer running

Still TODO:
- [ ] fix webui for new system (I'm thinking entry point moves to be `flexget webui` rather than its own script
- [ ] consider best api for plugins to register their subcommands (might be okay already, dunno)
- [ ] make sure all unit tests still work
- [ ] remove movie_queue force option (CLI for it is already gone)