Bug fix, small enhancement

Another bug fix. Issue #7 noted I'd failed to properly filter the tag data in flac -> mp3 conversions. This led to encoding errors and I assume failures. Solution was simple, add filter_item to mp3 tagging. Tested and resolved issue, confirmed fixed.

Small enhancement, since it was trivial to do, added AIF lossless type to supported list of types to convert to flac. Basically just extends the shn->flac logic by adding AIF. In testing, realized that also, ffmpeg transfers the tags from AIF to FLAC seamlessly, so that was also nice to see.