Ability to override just "display" for default button set

Using deepExtend instead of angular.extend on `$rootScope.textAngularTools` to allow easy customization of `display` for default buttons, without re-supplying the `action()` function

Eg, to ovveride 'Toggle HTML' `<button>` with `<a>` and custom icon:

```js
$rootScope.textAngularTools = {
      html: {
        display: "<a ng-click='action()' title='Toggle HTML'><i class='icon html'></i></a>"
      }
};
```