Make `json` filter bi-directional

I do this sort of thing pretty often:
http://bigbluehat.github.io/vue-uri/
(Output JSON into a textarea and react to it's changes).

So far, that's been with custom code. Would be very handy to have this be "native."

Thanks for considering this @yyx990803! :smile: 
Might want to add a try...catch block here and return the oldValue if there's an error parsing the value.