Allow to provide a custom template to ngtoast.

We had a need to customize the embedded toast container, so we made it possible to provide it from outside. 
Example:
<ng-toast template="ngtoast.html"/>

<script type='text/ng-template', id='ngtoast.html'>
  <ul class='ng-toast__list'
    <ng-toast-message ng-repeat="message in messages", message="message">
    <span ng-bind-html="message.content">
  </ul>
</script>