title: Best way to implement a toggle button in AngularJS Material
body: <p>I am looking for a simple solution to implement a toggle button with a custom selected / unselected icon in AngularJS Material.</p>
  <p>The functionality should be identical to the md-checkbox (with ng-model for the selection state), but I want to have my own icon displayed for selected / unselected state. md-checkbox does not seem to support custom icons, and md-button lacks the ng-model.</p>
  <p>Preferably I would like to avoid implementing a custom directive and only make this work through css. Is this possible with AngularJS Material?</p>