mixmonitor: Add option to enable a periodic beep

Add an option to enable a periodic beep to be played into a call if it
is being recorded.  If enabled, it uses the PERIODIC_HOOK() function
internally to play the 'beep' prompt into the call at a specified
interval.

Review: https://reviewboard.asterisk.org/r/3424/
monitor: use app options parsing helper code

This app is pretty ancient, so it was never converted to use the
option parsing helper code.  I'd like to add an option to this app
that takes an argument, and that's a pain to do when not using this
helper, so start by doing this conversion.

Review: https://reviewboard.asterisk.org/r/3429/