Forced mummification based on categories.
introduced incremental mummification, with a --full flag to force mummification of all artifacts. Now that with image mummification in GUISE-177, the images may not change often and we may want to force mummifying all the pages without reprocessing the images if they haven't changed.
Add an option --every that takes a list of categories. Eventually we may have this look at some category in the description, but for now simply query the artifact mummifier to see whether the artifact is in the category. Initially we can just have a few categories built in:
media (initially equivalent to image, but in the future will include audio and video)
(The idea here of "category" is like a "tag" or a "type", but of course "type" is too overloaded. The actually term doesn't have to be nailed down yet, as the CLI option will be --every).