Support for get_attached_file()

Resolves #70 
Resolves #162 

The filter will return the S3 URL when the local file is removed from the server. Some processes like regeneration of thumbnails need the physical file present on the server, so we allow this behaviour to be hooked into on a case by case basis. 
We should probably move this up higher and `return $url;` after logging the error. No point downloading the file if the filesystem isn't working.