Modify files before storing them

It would be useful to have the ability to modify files as they are uploaded. A simple example would be resizing an image using `gd`.

My initial thoughts are the `fileFilter` function could return the file as the second argument rather than a boolean. That way it could be manipulated in the `fileFilter` function and then be passed on to be stored, or dropped if false was passed instead of the file.

https://github.com/expressjs/multer/blob/master/lib/make-middleware.js#L94

This would be more useful when using an external storage engine such as `S3` as you wouldn't have to `upload -> download -> edit -> re-upload`.