fix(button): add a category to buttons so they won't get the button styles

fixes #6237

ionic/components/button/button.ts

@@ -58,6 +58,11 @@ export class Button {
    */
   isItem: boolean;
 
+  /**
+   * @input {string} The category of the button.
+   */
+  @Input() category: string;
+
   /**
    * @input {string} Large button.
    */
@@ -179,6 +184,16 @@ export class Button {
     this._readAttrs(element);
   }
 
+  /**
+   * @private
+   */
+  ngOnInit() {
+    // If the button has a role applied to it
+    if (this.category) {
+      this.setRole(this.category);
+    }
+  }
+
   /**
    * @private
    */