allow changing $schema in resource subschemas

[karenetheridge]

As explicitly described in json-schema-org/json-schema-spec#914 -- examine $schema wherever it (legally) appears and adjust evaluation semantics accordingly (including turning on/off keyword support via $vocabularies).

But as per json-schema-org/json-schema-spec#808 (comment) --

... implementations at least SHOULD check $schema and if available $vocabulary, but are free to error out on unknown or known but unsupported (e.g. old drafts) values.

(Support for earlier drafts' semantics may be covered in a separate issue, to be implemented separately.)

[karenetheridge]

blocked on #13 -- until then, we don't do anything special with the $schema keyword anyway.

[karenetheridge]

As of version 0.513 we now accept different values in the $schema keyword.