Enhance ExternalIVR with new options and commands.

(closes issue #12705)
 Reported by: ctooley
 Patches: 
       new_externalivr_argument_format-v2.diff uploaded by ctooley (license 136)
       new_externalivr_documentation.diff uploaded by ctooley (license 136)
       and a few additional fixes by me


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@117725 65c4cc65-6c06-0410-ace0-fbb531ad65f3