show: Introduce mime_node formatter callback

This callback is the gateway to the new mime_node_t-based formatters.
This maintains backwards compatibility so the formatters can be
transitioned one at a time.  Once all formatters are converted, the
formatter structure can be reduced to only message_set_{start,sep,end}
and part, most of show_message can be deleted, and all of
show-message.c can be deleted.