(rmail-mime): New custom group.

(rmail-mime): New custom group.
Move all defcustoms in this file into this group.
(rmail-mime-media-type-handlers-alist): Revert previous change.
(rmail-mime-show-images): New option.
(rmail-mime-total-number-of-bulk-attachments): Remove variable and all
references to it, since it wasn't actually used for anything.
(rmail-mime-insert-image): New function.
(rmail-mime-image): Use rmail-mime-insert-image.
(rmail-mime-bulk-handler): Remove optional `image' argument, instead
obey the value of `rmail-mime-show-images' option.