Support JSON-LD @context and (possibly) jsonschema in CLI

[wyc]

We should support @context and possibly jsonschema as options fed into the CLI. @clehner and I encountered this request at this most recent IIW. Perhaps it looks like:

# adding single JSON-LD schema for "http://w3id.org/mycontext"
$ didkit -@ "https://w3id.org/mycontext":mycontext.json vc-verify-credential ...

# adding multiple JSON-LD schemas for "http://w3id.org/context1" and "http://w3id.org/context2"
$ didkit -@ "https://w3id.org/context1":mycontext1.json -t "https://w3id.org/context2":mycontext2.json vc-verify-credential ...

# adding zero or more JSON-LD schemas for "http://w3id.org/places/country" and "http://w3id.org/places/state"
$ tree contexts
contexts
└── places
    ├── country
    └── state
$ didkit -@ "https://w3id.org/":./contexts vc-verify-credential ...

These are just some ideas. There's a lot of room for improvement, so feel free to take any design liberties to improve developer ergonomics and reduce chances of errors. Thank you @bumblefudge for the -@ flag suggestion. :)

@clehner, did you say there was another project that used this approach?