Working prototype for extensions (one way of doing them)

Needs a new `npm install` -- adds a JSON-pointer and lodash dependency.

The idea for this prototype is to analyze the assertion and generate a list of "directives" to run for schema validations. This includes the root assertion against the core schema document as well as a generated directive for each properly formed and declared extension.

## TO DO
TODO: needs a fallback for 1.0 and other assertions that don't declare `@context` like this
TODO: needs to fetch remotely linked badge objects and substitute them into the existing structure. The jsonpointer.js library might be helpful for the second half.