Visualize - Choose first field by default

Closes #2215

This seemed to be the only select input that made sense to set a default for.
This is a recreation of the `ng-options` logic in the view. I hate that I have to pass it in this way, but `<vis-agg-param-editor>` has an isolate scope, and you can't pass objects as parameters. I could remove the isolate scope, but I'm wondering if there's a better way to handle this...