Add send DTMF feature to ExternalIVR app

Implemented a new command 'D' that allows client
IVRs to send DTMF digits to the channel.

(closes issue #16615)
Reported by: thedavidfactor

Review: https://reviewboard.asterisk.org/r/465/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@242357 65c4cc65-6c06-0410-ace0-fbb531ad65f3