skip non-audio when scanning library for new content

I have a number of non audio files interspersed in my audio library: cover images, lyrics... And every time I start Groove Basin, it tries to add those to the library, slowing down the startup process and generating a number of warnings/errors. This patch adds a quick MIME type based check to filter and scan audio content only.

N.B.: the [mime](https://github.com/broofa/node-mime) library check is extension based only. If a content check is preferable then one option would be to use  [mmmagic](https://github.com/mscdex/mmmagic).