func_beep: New function for periodic beeps.

This commit introduces a new dialplan function, BEEP().  It allows you
to enable playing a beep tone at a regular interval into a call.  The
most common use case for this is to use during call recording to
notify and remind the callers that the call is being recorded.  A
future commit will update the call recording applications in Asterisk
to use this automatically if an option is specified for convenience.

The implementation makes heavy use of existing Asterisk components.
Instead of replicating logic required to load a sound file, transcode
it if necessary, and do audio mixing, it makes use of
Playback+ChanSpy to accomplish the task.

The other important bit of the implementation is how it figures out
when to trigger the beep playback.  This implementation uses the
audiohook API, even though it's not actually touching the audio in any
way.  It's a convenient way to get a callback and check if it's time
to kick off another beep.  It would be nice if this was timer event
based instead of polling based, but unfortunately I don't see a way to
do it that won't interfere with other things.