Filter feed items by media type/extension

[tkem]

From [https://www.apple.com/itunes/podcasts/specs.html#common-tags]:

The file extension of the URL attribute of this tag is used to determine if an item
should appear in the podcast directory. Supported extensions include .m4a, .mp3,
.mov, .mp4, .m4v, .pdf, and .epub.

[...]

The type element depends upon the type of file the enclosure refers to. Common
files and their MIME type extensions are listed in the following table.

File	Type
.mp3	audio/mpeg
.m4a	audio/x-m4a
.mp4	video/mp4
.m4v	video/x-m4v
.mov	video/quicktime
.pdf	application/pdf
.epub	document/x-epub

[tkem]

Scheduled for next release, since .pdf and .epub don't make sense with Mopidy.