Can now import covers from track metadata and track directory as well

Until now the ID3Tag for the Track-Number was read from TPOS.
The Problem is, that TPOS is the part of the set (eg disc number)
and not the track-number. TRCK is the correct tag for the track
number.

Fixes #220

OggTheora wasn't support up until now. Now the config is there and the
import works

fixes #216

If you ever need an empty flac file with metadata again:

1. Get a flac file (like https://archive.org/download/NineInchNailsTheSlip24bit96khz/01999999.flac)
2. Tag it with Musicbrainz Picard
3. Truncate it, keeping only tags with `ffmpeg -i in.flac -ss 0 -to 0.001 out.flac`

Thanks @HgO for the trick!

Fixed #21: can now import MP3 files via command line, also improved musicbrainz ID handling in import process