Adding add_filter('post_thumbnail_html'...)

This plugins great and works really well for what I'm after when used within 'the_content()' but didn't responsify images using 'the_post_thumbnail'. To get that working I've added another filter in directly below 'the_content' to filter post thumbnail html too.

For me this makes sense to add into the plugin, but your shout of course.

FYI: When using this in my theme, I use the following:
```
<?php the_post_thumbnail('full',
  array( 'data-responsive' => get_post_thumbnail_id(),
  'class' => "size-full wp-image-" . get_post_thumbnail_id()
) ); ?>
```