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