Ng model

Here's yet another take on it. In this branch I'm having angular-knob be a legit angular custom form control by using ng-model. The require attribute let's me pull in the ngModelController from the the ng-model directive on this same element. https://docs.angularjs.org/api/ng/type/ngModel.NgModelController describes it in detail, but basically this gives me access to the angular form component api, I can participate in validation, there's a rendering and formatting pipeline if I need that, all kinds of stuff.