Add nested schemas

[audse]

Here's an example of what I'd like to support:

{
    "$schema": "http://json-schema.org/draft-07/schema",
    "definitions": {
        "Resource": {
            "type": "object",
            "properties": {
                "resource_path": {
                    "type": "string"
                }
            }
        }
    },
    "type": "object",
    "properties": {
        "some_resource": { "$ref": "#/definitions/Resource" },
    }
}

This schema will cause the some_resource property to load the resource at Resource.resource_path.