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 template-url="custom_tab_template">
      <tab heading="Stuff">
        Some stuff here
      </tab>
    </tabset>