Initial support for ALAC encoded audio files

Both AAC and ALAC are normally embedded in an .m4a container. For
this reason I have split the m4a type into aac and alac types.

To distinguish between the two encodings I have taken advantage
of the fact that, in my collection, ALAC don't have a sample_rate
set. I could not find verification if this is always the case
or not. Would bitrate be a more reliable determining factor?
e.g. if bitrate > 320000: type = 'alac'

Signed-off-by: Simon Luijk <simon@simonluijk.com>

Initial issue #142