Allow specifying indentation amount in json filter

[`json`](https://docs.angularjs.org/api/ng/filter/json) filter indentation amount is hard coded to 2 space. Please allow specifying the indentation amount. I see [indentation for global `toJson`](https://github.com/angular/angular.js/blob/d3b1f502e3099d91042a1827a006ad112ea67d36/src/Angular.js#L972) is hardcoded to 2 spaces too.  Maybe it requires changing that.
