mod_callcenter: style
mod_callcenter: support cc_exit_keys channel variable

If any digit in cc_exit_keys is pressed, the member will exit the
queue and the callcenter application.
mod_callcenter: set cc_exit_key channel variable

If a member of a queue presses a digit specified in cc_exit_keys, the
caller will exit the queue and the variable cc_exit_key will be set to
the digit the caller pressed.
mod_callcenter: implement cc_outbound_announce

Playback one or more things to the agent based on a channel variable
of the member.  cc_outbound_announce may contain an array.
mod_callcenter: add cc_outbound_cid_name_prefix

Prefixes the displayed caller_id_name when calling outbound agents
without affecting the caller_id_name of the underlying member channel.