Auto set json flag on config.set

[fbaiodias]

Currently you can't call config.set with an object as the value. You need to JSON.stringify it and pass { json: true } on the opts.

I suggest that we check the type of the value, JSON.stringify it if it's an object set the opts automatically if none were provided.

Let me know what you think, I would be happy to PR this :)

[dignifiedquire]

Yes please!