feat(menu): opening event EventEmitter

Closes #632

ionic/components/menu/menu.ts

@@ -202,6 +202,7 @@ export class Menu extends Ion {
       this._prevent();
       this.app.setTransitioning(true);
       this._getType().setProgess(value);
+      this.opening.next(value);
     }
   }
 

ionic/components/menu/test/basic/index.ts

@@ -58,4 +58,8 @@ class E2EApp {
       this.app.getComponent('leftMenu').close();
     });
   }
+
+  onMenuOpening(ev) {
+    console.log('onMenuOpening', ev);
+  }
 }

ionic/components/menu/test/basic/main.html

@@ -1,4 +1,4 @@
-<ion-menu [content]="content" id="leftMenu" side="left">
+<ion-menu [content]="content" id="leftMenu" side="left" (opening)="onMenuOpening($event)">
 
   <ion-toolbar secondary>
     <ion-title>Left Menu</ion-title>