Improved stt module

I've improved the STT module.

Notable changes:
- STT engines inherit from `AbstractSTTEngine`
- Use a TranscriptionMode enum instead of `MUSIC` and `PERSONA_ONLY` argument in `transcribe()`
- Use slugs (`sphinx` and `google`, so that the `profile.yml` does not have to be edited)
- Move configuration logic into the respective class (see `AbstractSTTEngine.get_config()`)
- Make lm/dic pairs of `PocketsphinxSTT` configurable via `profile.yml`.