Scope for keys in JSON objects

I noticed [this](https://github.com/textmate/json.tmbundle/pull/2) PR was also trying to address this issue, and you mentioned preferring a string scope. Is this closer to what you had in mind?

Proposal is because you need a [nasty hack](https://github.com/MattDMo/Neon-color-scheme/blob/master/Neon.tmTheme#L1139) to highlight keys in the current syntax.