Replace JSON validator with standard one based on JSON schema

[symposion]

From @symposion on April 6, 2016 8:34

Ideally I'd like to move the whole spec format to JSON schema, but I suspect that it won't handle the complexities of the layout I need to deal with the text parsing. Failing that, we should generate JSON schema and use a standard validator instead of the custom code that I have at the moment.

Copied from original issue: symposion/roll20-api-scripts#52