Update configs to allow for return to main/exit and repeat menu

Adds configuration information necessary for adding the following DTMF buttons:

'*' - repeats the last menu.
'#' - exits from the change folders prompt. If already in the main menu, plays bye and hangs up.
After reviewing the code in asterisk/node-voicemail-main-fsm, it looks like that will actually work as intended but I still think it's confusing from the point of view of the config.

I think we should think about this some more to see if there's a better way to define the config that is less confusing and works for all cases (I'm sure this would involve slightly modifying the asterisk/node-voicemail-main-fsm dtmf handler code).