add @context to jsonld serialization

[bourgeoa]

Actually the serialize display of jsonld is not easily readable by humans. The idea is to add a @context that will includes the equivalent to turtle @prefix.

[TallTed]

@bourgeoa -- Please revisit the title and initial comment on this issue, and code fence each instance of @context and @Prefix by wrapping them in backticks (like `@context` and `@Prefix`), so that those GitHub users don't get pinged on every update to this issue which they are not participating in of their own volition.

[bourgeoa]

Thanks.