title: Validate against schema fragment
body: <p>I'm new to json-schema so it might not be a relevant issue.
  I'm using <a href="https://github.com/hoxworth/json-schema" rel="nofollow">https://github.com/hoxworth/json-schema</a>.
  I have one big json file describing a lot of schemas (mostly small ones) with a lot $ref between schemas, and I need to be able to validate data against one of these "inner" schemas. I can't find a way to do this with json-schema.
  Does json-schema support this use case, or am I doing it wrong ?</p>