Allow full JSON-LD @context

[nichtich]

As discussed in #119, the value of @context should be allowed to be any valid JSON-LD context. In particular this allows to define custom fields like this:

{
  "@context": {
    "@import": "https://gbv.github.io/jskos/context.json",
    "CUSTOM": "http://www.example.org/customUri"
   } ...

[nichtich]

Done with 0.5.3 (Allow @context to hold an array)