Lightbox service: support custom inline templates

[victor-o-silva]

Support custom inline templates in addition to custom template paths.

angular.module('app').config(function (LightboxProvider) {
  // set a custom template
  LightboxProvider.template = '<h2>Customized</h2><img lightbox-src="{{Lightbox.imageUrl}}"><h2>lightbox</h2>';
});