Add the ability to continue and originate using priority labels.

With this patch, the following two ARI commands

POST /channels
POST /channels/{id}/continue

Accept a new parameter, label, that can be used to continue to or originate
to a priority label in the dialplan.

Because this is adding a new parameter to ARI commands, the API version of
ARI has been bumped from 1.6.0 to 1.7.0.

This patch comes courtesy of Nir Simionovich from Greenfield Tech. Thanks!

ASTERISK-24412 #close
Reported by Nir Simionovich

Review: https://reviewboard.asterisk.org/r/4285
........

Merged revisions 430337 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430338 65c4cc65-6c06-0410-ace0-fbb531ad65f3