Added a feature to specify the fields to output

Hey Matt

I've added a feature to allow the fields to be output to be specified as a list of strings. I know your normal policy is to raise an issue first, but we needed this feature quickly and I've already implemented it. 

I wanted to get your feedback on it first because I know it might not the the right "fit" for the library. If you're interested in the feature I'll follow the rest of the feature request process and update the documentation + make and modifications, etc.

Thanks
Dylan