Simplify logging

[jodal]

See #1452 for background and reasoning.

[kingosticks]

This looks really good, will be very useful I am sure.

[adamcik]

This doesn't work as we send logs to stderr, you need mopidy -vvv 2>&1 | tee mopidy.log

[jodal]

Or, we could create the StreamHandler with stream=sys.stdout?

I guess using stderr for logging and stdout for console output/interaction, like for mopidy local scan is the proper way to do it, so we should probably just update the docs.