Add `contains` keyword

[BigBlueHat]

Just surfacing this proposal from https://github.com/json-schema/json-schema/wiki/contains-%28v5-proposal%29

It would greatly simplify JSON Schemas such as this one:
https://github.com/w3c/web-annotation-tests/blob/master/common/contextValue.json#L15-L26

It has been implemented in ajv.

/cc @awwright @epoberezkin

[handrews]

@BigBlueHat , @awwright : I just realized I re-filed this as issue #63 as I did not think anyone had migrated it over. Since I copied the full text of the proposal in that issue, can I request that we close this one and keep #63 ? Or I could copy stuff over here.

[handrews]

Done! Please close unless we missed something (I don't have the permissions to close this).

[Relequestual]

Closed as merged.