Allow "$schema" via schema.json

The schema should allow the “$schema” element in the referencing document.

Sample Document

{
  "$schema": "https://raw.githubusercontent.com/OAI/OpenAPI-Specification/master/schemas/v2.0/schema.json",
  "swagger": "2.0",
  "info": {
    "title": "My Service API",
    "version": "0.0.1"
  },
  "paths": {}
}

“Property $schema is not allowed.”

{
	"resource": "/Users/gpalacino/Source/play/service/service-api/openapi.json",
	"owner": "_generated_diagnostic_collection_name_#0",
	"code": null,
	"severity": 4,
	"message": "Property $schema is not allowed.",
	"startLineNumber": 2,
	"startColumn": 3,
	"endLineNumber": 2,
	"endColumn": 12
}

[handrews]

$schema is a JSON Schema keyword, and OAS is not, itself a JSON Schema (it is a document that contains JSON Schemas within it). While some people do use a $schema property in JSON documents the same way that JSON Schema uses it, this is not actually any sort of standard and is, IMHO, more confusing than helpful.

[handrews]

[EDIT: I misread this and didn’t realize it was about putting $schema at the root of OAS itself. I’ve taken out what I wrote before and will put that in a separate issue.]