Add ability to fetch data directly from URL's

[larsiusprime]

You should be able to specify an ArcGIS endpoint along with the usual filename.

• If a flag is set, or no local file is present, it fetches from the URL, and then writes a cached version to disk (which is the filename you supply)
• If a flag is not set, and a local file is present, it loads from the local file
• Local file will get synced to cloud