Added non editable fields support & classes on variables

[sokolbird]

Hi! We are using your package in our projects and we found out that we need a feature which is not available out of the box. We need to allow editing on the whole json, but disable it for some separate fields. That's what this pull request is about. Also added classes which represent field names in order to make css modifications easier.

[byuzyfyshynAmplify]

@mac-s-g , it seems like adding this feature would be useful for all users. Could you have a look at it?

[galr52]

hey i have created a fork because this repo is not active, you can open your or there https://github.com/galr52/react-extreme-json-view