allow pasting of URL instead of JSON body

[konklone]

fetched with CORS, error out if it doesn't work.

specifically, don't try JSONP

[konklone]

for advanced mode only

[danmunz]

Now that I read it, I suppose this could take the place of #24.

[konklone]

Well, there's so much out there that doesn't support CORS (or JSONP), I don't know how reliable this approach would be. I'd like to introduce this into the UX for advanced users who won't get scared by the use of the term "CORS". I don't know if it'd replace #24 either way.