Allow x-* keys for extensions

[dhouck]

I am considering doing a basic dependency management script while packwiz/packwiz#97 isnʼt complete. It would be nice to be able to use keys for storing that information which were guaranteed to not conflict with official keys, and making non-official sources start their key names with x- is a standard way to do that.

I donʼt know if jsonschema supports that or if it would need to be in prose, but either way, it would be useful.