feat(tab): make templateUrl configurable

I need the ability to have different markup for some tabs on a page
by page basis. Injecting templates with the script tag on each page
is getting ugly.

This topic was recently brought up again in #1611.

@sudhakar mentions a similar need for this in #105.

Usage

<tabset>
  <tab ng-repeat="tab in tabs">
    <tabset template-url="custom_tab_template">
      <tab ng-repeat="innerTab in tab.tabs">
        <span class="inner-tab-content">{{ innerTab.content }}</span>
      </tab>
    </tabset>
  </tab>
</tabset>