Extended with tiled background images

Extends `background_fill` with an extra argument `tile`.

Usage:
Set `background_fill` to `tile` (eg `background_fill=tile`)
and
`tile` to an image path or url (eg `tile=http://domain.ext/image.jpg).


Example:
```
the_post_thumbnail( 'width=650&height=300&crop=1&resize=true&background_fill=tile&tile=path-or-url-to-tile-image' );
```

Please review before adding. Works for me but was coded quite fast.