JSON Preprocess function for things like nested props

[micahgodbolt]

Still need to write up tests/docs, but wanted to make sure the placement and name of this function works for you. I was able to get my nested props function working just fine with this.

      jsonPreProcess: function (json) {
        let resolveNestedProps = function (props, parent = "") {
          let flatProps = {};
          _.forEach(props, (value, key) => {
            if (typeof value !== 'string') {
              let propName = (parent === "" ? "" : parent + "_") + key
              flatProps = Object.assign(flatProps, resolveNestedProps(value, propName))
              flatProps[propName] = value
            }
          })
          return flatProps;
        }
        json.props = resolveNestedProps(json.props);
        return json;
      }

Might consider dropping this into a small NPM module if others would like to easily use it.

[kaelig]

Could we could use a fat arrow function? We're using ES6 throughout the examples so it might make sense to be consistent.

jsonPreProcess: (json) => {

[kaelig]

This is brilliant, thanks a lot!! I just had one comment and we're good to go

[micahgodbolt]

Forgot that this closes issues #64