feat: add [iron-focusable] to mark as focusable

Author: Elliott Marquez

README.md

@@ -34,7 +34,8 @@ options.
 
 In addition, `with-backdrop` will wrap the focus within the content in the light DOM.
 Override the [`_focusableNodes` getter](#Polymer.IronOverlayBehavior:property-_focusableNodes)
-to achieve a different behavior.
+to achieve a different behavior. Additionally, you can add the `iron-focusable` attribute to
+any element in the light DOM to mark it as focusable.
 
 ### Limitations
 

iron-focusables-helper.js

@@ -57,7 +57,9 @@ class IronFocusablesHelperClass {
     }
     // Elements that can be focused even if they have [disabled] attribute.
     return matches.call(
-        element, 'a[href], area[href], iframe, [tabindex], [contentEditable]');
+      element,
+      'a[href], area[href], iframe, [tabindex], [contentEditable], [iron-focusable]'
+    );
   }
 
   /**