Class Icon based for elements

[ignacio...@gmail.com]

Hi,

It would be great if we can uses class based icon such us icomoon or fontawesome. 

Right now we can do it with few elements embedding a <i> element inside the polymer element, but it does not work in all the elements for example paper-menu-button or paper-item

I propuse add a iconClass property  and use it as in core-item with the other icon properties

   <template if="{{iconClass}}">

        <span class="{{iconClass}} icon"></span>

    </template>

Best Regards,

Ignacio