amp-bind: Support <select>

[dreamofabear]

We should support setting state vars from drop-down menus. One way to do this is add support for <select>'s change event, as we currently do for <input>:

<select on="change:AMP.setState({event.value})">
  <option value="foo">foo</option>
  <option value="bar">bar</option>
</select>

/to @kmh287