Added ability to clean clipboard ✨ (#2468)

-  Added the possibility to clean the clipboard on Android, iOS and Windows.

src/@ionic-native/plugins/clipboard/index.ts

@@ -25,6 +25,8 @@ import { Cordova, Plugin, IonicNativePlugin } from '@ionic-native/core';
  *       alert('Error: ' + reject);
  *     }
  *   );
+ * 
+ * this.clipboard.clear();
  * ```
  */
 @Plugin({
@@ -52,4 +54,11 @@ export class Clipboard extends IonicNativePlugin {
   @Cordova()
   paste(): Promise<any> { return; }
 
+  /**
+   * Clear the text stored in clipboard
+   * @returns {Promise<any>} Returns a promise after the text has been cleaned
+   */
+  @Cordova()
+  clear(): Promise<any> { return; }
+
 }