Override handle query result

I wrote my own dataset for use with a recline model.  The results from this dataset don't exactly match the format expected by _handleQueryResult in src/model.js, so I needed a way to do my own processing to get them into the model.

I added a few lines to src/model.js to check if a dataset has its own _handleQueryResult, and if so, use that instead of the default method.  I think there might be a better way to do this but I'm not enough of a JavaScript expert to see it.  It does seem like a not-unusual use case though.  Please let me know if you want more information about how I'm using this.