Robustly support schema references via URL

[joelgallant]

Detect and resolve $ref properties that point to http/s URLs. This is useful for databases like schemastore.

This would be implemented in schema.ts, in resolveExternalSchemas. Detect URLs like we detect filepaths, fetch JSON and add it to schema references in the same way. Have to do this recursively as well.

[joelgallant]

#53 basically provides this out of the box. It's not exactly plug-n-play though (for example, non-text Content-Types don't tend to work). Would probably have to play with the http options of json-schema-ref-parser.