Add an announcement option to music-on-hold - plays sound when put on hold/between songs

This is a feature patch which allows an 'announcement' option to be specified in
musiconhold.conf which should be set to the name of a sound. If a valid sound is
specified for this option, then it will be played on that music on hold class whenever
a channel bound to that class is put on hold as well as when Asterisk is able to detect
that a song has ended before starting the next song (excludes external players).

(closes ASTERISK-18977)
Reported by: Timo Teräs
Patches:
	asterisk-moh-announcement.diff uploaded by Timo Teräs (license 5409)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@352134 65c4cc65-6c06-0410-ace0-fbb531ad65f3