Makes 'dialplan add extension' create the specified context if it does not already...

Makes 'dialplan add extension' create the specified context if it does not already exist.

If the user invokes 'dialplan add extension' into a non-existing context, the context will be created
and a message informing the user of the context being created will be issued in cli.

(closes issue #17431)
Reported by: leearcher
Patches:
      context_auto_create.diff uploaded by kobaz (license 834)
Tested by: leearcher, kobaz, jrose

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@312678 65c4cc65-6c06-0410-ace0-fbb531ad65f3