Accept "arbitrary" URLs

[Xion]

Instead of just canned URLs to gists, we could accept "any" URL that points to code. At first this could be just plain text code (like raw versions of files hosted in regular repos on GitHub/BitBucket/Gitlab/etc.). V2 may also accept HTML browser URLs where we'd try to find the code based on some heuristics.

Internally, http could function like a virtual gist Host. The Uri would have to be expanded to essentially allow arbitrary URLs and not just <owner>/<name>. Or we could introduce a more general gist moniker (essentially a more advanced version of args::GistArg).