Add colorbox support

It would be nice to add colorbox support for Lazy module. For example if we have a gallery with a big amount of images, that rendered by colorbox, browser tries to load all of thumbnails. Now there is no way to enable Lazy load for colorbox, but I made patch for this.

I just added simple function, that returns an array of supported widget types:

function _lazy_supported_fields() {
  return [
    'image',
    'colorbox',
  ];
}

and changed conditions like this:

if ($element['#field_type'] === 'image') {

to this:

if (in_array($element['#field_type'], _lazy_supported_fields(), TRUE)) {

And it works fine.