add JSON Schema generator

[onyxraven]

https://github.com/chrusty/protoc-gen-jsonschema

I am happy to put together a PR for this as well, it should be able to follow a pattern like others, though its way of configuring options is a little weird, and I'll have to think about if that explodes the list of available options on the command.

[ido-namely]

Hi @onyxraven ,
PRs are still welcome :)
I suppose you can expose an argument similar to --with-docs which takes all possible inputs and pass them to the plugin.