af: add metadata field to af_stream and af_instance

This allows to propagate metadata information to audio filters.

Closes #632
af_volume: add replaygain support

This adds the new options replaygain_track and replaygain_album. If either is
set, the replaygain track or album gain will be automatically read from the
track metadata and the volume adjusted accordingly.

This only supports reading REPLAYGAIN_(TRACK|ALBUM)_GAIN tags. Other formats
like LAME's info header would probably require support from libav.