feat(label): add label attribute binding

Author: serifine

src/checkbox/ux-checkbox.html

@@ -13,5 +13,5 @@
   </div>
 
   <!-- Leave Inline, uses :not(:empty) selector to determine margin -->
-  <label ref="label"><slot></slot></label>
+  <label><slot>${label}</slot></label>
 </template>

src/checkbox/ux-checkbox.ts

@@ -12,6 +12,7 @@ import { processDesignAttributes } from '../designs/design-attributes';
 export class UxCheckbox implements Themable {
   @bindable public disabled: any = null;
   @bindable public effect = null;
+  @bindable public label: string;
   @bindable public matcher = (a: any, b: any) => a === b;
   @bindable public model: any;
   @bindable public tabindex = 0;