Allow jsonSchemaId to be passed separately

Created by: nicksnyder

This is part of https://github.com/sourcegraph/sourcegraph/issues/914 and a pre-requisite for https://github.com/sourcegraph/sourcegraph/pull/1103

It will allow me to validate a config against a definition in site.schema.json like this:

jsonSchemaId="site.schema.json#/definitions/GitHubConnection"
extraSchemas={[siteSchemaJSON]}