feat(buttons): dynamically add button attributes

Closes #5186

ionic/components/button/test/clear/index.ts

@@ -4,4 +4,10 @@ import {App} from 'ionic-angular';
 @App({
   templateUrl: 'main.html'
 })
-class E2EApp {}
+class E2EApp {
+  clearButton = true;
+
+  toggleClear() {
+    this.clearButton = !this.clearButton;
+  }
+}

ionic/components/button/test/clear/main.html

@@ -35,4 +35,8 @@
     <button clear dark class="activated">Dark.activated</button>
   </p>
 
+  <p>
+    <button [clear]="clearButton" (click)="toggleClear()">Toggle Clear</button>
+  </p>
+
 </ion-content>