Allow style and data attributes in formatter (#57)

[blms]

Hi @rsimon! Here's a PR to address the first two bullet points of #57.

Working with a client (eScriptorium) that uses recogito-js and needs colors pulled from an API endpoint dynamically applied to annotations, so we need to be able to pass the custom style attribute like in Annotorious.

The comments and some code were cribbed directly from Annotorious 😄

[rsimon]

Hi @blms, looks great - thanks!

Great to hear that eScriptorium is using RecogitJS, BTW. I think I remember having discussions with them about it (quite a while back though). I'd be interested in hearing more about the type of content they are using it with. Also in terms of whether it might be possible to upgrade them to the successor library at some point?

[blms]

@rsimon They are using it with HTR transcription content, mostly (I believe) to mark metatextual things like insertions in marginalia. We will recommend the successor library upgrade to them!