Feature: Extract metadata and export them to stats.xml

All metadata is now extracted from Vorbis streams and ICY streams
and exported as new <metadata> tag into admin/stats if specific
mountpoint is requested.

To testers:
 - Please test with Ogg Vorbis streams.
 - Please test with Ogg Vorbis + Theora streams.
 - Please test chained Ogg streams.
 - Please test with ICY streams.

close #2106