Supporting choice on a canvas

[atiro]

I'd like to generate manifests with prezi3 using choice to enable multiple images to be stacked on one canvas (Recipe 0033) . As far as I can see at the moment this can't be done as there is no way to set the type of Choice in the annotation (but I might be not reading this correctly)

I had a quick attempt at adding this which seems to work (https://github.com/atiro/iiif-prezi3/pull/1/files) but is hardcoding to only the first canvas. I'll keep working on it if this is a correct approach, otherwise happy to be told of a much better way.

[glenrobson]

This doesn't address your proposal for a helper method but does this recipe help to detail how to use choice:

#214