Add aria-label for button-toggle

Author: tinayuangao

src/lib/button-toggle/button-toggle.html

@@ -5,6 +5,8 @@
          [checked]="checked"
          [disabled]="disabled || null"
          [name]="name"
+         [attr.aria-label]="ariaLabel"
+         [attr.aria-labelledby]="ariaLabelledby"
          (change)="_onInputChange($event)"
          (click)="_onInputClick($event)">
 

src/lib/button-toggle/button-toggle.ts

@@ -277,6 +277,17 @@ export class MdButtonToggleGroupMultiple extends _MdButtonToggleGroupMixinBase
   }
 })
 export class MdButtonToggle implements OnInit, OnDestroy {
+  /**
+   * Attached to the aria-label attribute of the host element. In most cases, arial-labelledby will
+   * take precedence so this may be omitted.
+   */
+  @Input('aria-label') ariaLabel: string = '';
+
+  /**
+   * Users can specify the `aria-labelledby` attribute which will be forwarded to the input element
+   */
+  @Input('aria-labelledby') ariaLabelledby: string | null = null;
+
   /** Whether or not this button toggle is checked. */
   private _checked: boolean = false;