Use name of processing directive as original_name attr of emitted file.

[grosscol]

For the default processors: image, audio, video, document

Pass the name of the directive responsible for the derivative (e.g. thumbnail) as the original_name on the derivative file that gets emitted to output_file_service.call. This can be a convenience to downstream uses. The original_name is currently set to "derivative" and was previously nil.

The original_name attribute of the file passed to output_file_service.call should be the name of the directive that caused the file to be generated. For example:
transform_file :original_file, thumbnail: { format: 'jpg', size: '338x493' }
should eventually pass a file to the output_file_service that has the attribute original_name set as "thumbnail".

Related to #70.