Add support for JSON-LD

The result will be a valid JSON-LD context.
Introduce new route "/context" (with optional extension)

This makes it easier to remember the URL for the JSON-LD context. It will be

    http://prefix.cc/context

or

    http://prefix.cc/context.jsonld

instead of

    http://prefix.cc/popular/all.file.jsonld
Merge pull request #15 from lanthaler/json-ld-support

Add support for JSON-LD contexts