Generate IIIF presentation manifests separately

[bmschmidt]

Feature request/suggestion, curious about your thoughts because I can image some potential pitfalls.

wax:derivatives:iiif does multiple things at once.

1. A very expensive process of creating tiled image sets
2. A very cheap process of creating IIIF image API v. 2.0 compliant JSON files.
3. A very cheap process of creating IIIF presentation manifests version 2.0

It would be useful to be able to run the third process without the first two, because:

1. This would open the door to including and updating the CSV metadata inside the IIIF presentation manifest, which is a logical step for making collections more reusable. (Will file that separately as an issue).
2. It would make it easier to build a wax site around images (including crops of images) hosted elsewhere on the web. (For instance, union collections of library of congress and internet archive collections, or faculty setting up rogue IIIF servers for grad students b/c they can't get permanent ones out of their library RT division).

I would envision this as a new task called wax:derivatives:manifests. (Although the naming there is a little odd, because I'm proposing only presentation manifests, not image manifests. I think.)