Add options PreDial options 'b' and 'B' to app_dial

* Added 'b' and 'B' options to Dial.  These options will allow you to run
  last-minute dialplan on the caller and callee channels while the Dial
  application is executing, but before the call is started.  For example you
  can use the 'b' option to run dialplan on the callee channel to get the name
  of the newly created channel right away.

Review: https://reviewboard.asterisk.org/r/1229/

(closes issue: ASTERISK-19548)
Reported by: Mark Murawski
Tested by: Mark Murawski, Stefan Schmidt



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@359705 65c4cc65-6c06-0410-ace0-fbb531ad65f3