Add support for choice on canvases

[jamesmisson]

This PR adds support for choices on canvases, following on from IIIF-Commons/manifesto#173.

The Helper now has:

• choiceIndex to track choice state
• hasChoices
• getActiveChoice
• getChoices

And tests for the IIIF cookbook manifest.

I'll leave this in draft for now as it should be tested with a manifest with multiple canvases with choice.

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[demiankatz]

Thanks, @jamesmisson, and sorry it has taken me a couple of days to get back to you. See below for a couple of comments. Regarding the versioning piece, we've been pretty inconsistent about semantic versioning in the past, so a minor misstep here is not a disaster, but we should aim to be more consistent in future!

[jamesmisson]

Thanks for the review @demiankatz , I've made the recommended changes

[demiankatz]

Thanks, @jamesmisson, looks good to me! You still have this in draft mode, so I won't merge it yet... but feel free to take out of draft and merge if you're done!