Add JSON key scope as string.quoted.double.key.json

Right now, specifying JSON keys for syntax highlighting requires a [nasty hack](https://github.com/MattDMo/Neon-color-scheme/blob/master/Neon.tmTheme#L1139). I noticed [another PR](https://github.com/textmate/json.tmbundle/pull/2) which was declined, so I thought I'd give it a try.

I basically grabbed the entire `string` rule and c+ped it in place of the include, with the appropriate name based on the string scope, `string.quoted.double.key.json`. The c+p seemed clumsy to me, but I couldn't find any other way to do it. (I tried taking the name out of the `string` rule, and then specifying it at each of the points where it was included, but that didn't seem to work.) Let me know what you think.