app_dial: Expanded A option to add caller announcement

Hitherto, the A option has made it possible to play
audio upon answer to the called party only. This option
is expanded to allow for playback of an audio file to
the caller instead of or in addition to the audio
played to the answerer.

ASTERISK-29442

Change-Id: If6eed3ff5c341dc8c588c8210987f2571e891e5e