Ability to pull CLDR data from JSON

[zbraniecki]

As the first step to de-hardcode CLDR data in this crate, I'd like to allow for CLDR data to be loaded from CLDR json resources.

This is not the final solution, as we'll likely want to (also) allow for some more optimized resources, but having two modes - hardcoded and JSON CLDR, would be a good first step.

See: unicode-org/rust-discuss#5