Override LoadURL at Compiler

[jfancher]

I'm implementing a feature that will allow schemas to be stored and updated; the details aren't very relevant, but the key point is that a schema URL may not have a globally-consistent referent (for instance, the current value vs one being updated). Thus, I'm finding the global LoadURL function to be limiting as a resolution strategy.

It seems like a simple solution would be to make the loader function local to the Compiler object (and initialized to the global). Does this change make sense? If so, I would be happy to send a PR.

[santhosh-tekuri]

makes sense. you can send PR.