Consider adding JSON Schema support

[eemeli]

At the moment, the schemas that we support all come from the YAML spec, and specify generic types that may exist in a document. This means that for plain nodes in particular, implicit tags are assigned to values based on regexp pattern matches.

As a possible alternative, we could support something like JSON Schema instead, which would allow for defining a document-specific structural schema, which would permit for much greater type safety in our output.