Add support for multiple contexts

[Adri-7]

As we can see in presentation specification, we can give multiple contexts (to link default and custom contexts).

e.g

{
  "@context": [
    "http://iiif.io/api/presentation/2/context.json",
    "http://example.org/extension/context.json"
  ]
}

It seems that the context is only a string but it would be great if it was a collection :slightly_smiling_face: https://github.com/dbmdz/iiif-apis/blob/b926671f3cac2f9411969b566e6d134efbe53eed/src/main/java/de/digitalcollections/iiif/model/sharedcanvas/Resource.java#L39

[Angelthree95]

Hey, are you going to implement multiple contexts? I don't see it implemented in 0.3.9