Add support for having configuration inside package.json

[dlmr]

Would be great if it was possible to have the configuration inside package.json.

[sebinsua]

I'd prefer this. I don't know why the config is in roc.config.js by default. A 'roc' key within package.json would be preferable.

[dlmr]

If we implement this not all of the possible configuration options will be available since some things are not possible to do in JSON. With that said we should support this for everything that can be expressed in JSON.