Add a post_apply callback to the Config Options API

This adds a callback that only fires when changes have been successfully
applied via the Config Options API.

Review: https://reviewboard.asterisk.org/r/1980/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@368921 65c4cc65-6c06-0410-ace0-fbb531ad65f3