Add option imagemagick-types-enable

Add option imagemagick-types-enable

* lisp/image.el: For clarity, call imagemagick-register-types at
top-level, rather than relying on a custom :initialize.
(imagemagick-types-enable): New option.
(imagemagick-register-types): Respect imagemagick-types-inhibit.
If disabling support, remove elements altogether rather
than using an impossible regexp.
(imagemagick-types-inhibit): Give it the default init function.

* src/image.c (Fimagemagick_types): Doc fix.

* etc/NEWS: Mention this.