Propose a mechanism to auto-remove temp files on response close/finish

Add a second option to "multipart({}, true);" to trigger automatic temp files deletion.

It's a proposition and maybe the implementation is not totally compliant with Connect/Node standards.

I also wasn't able to run the repo tests (Uncaught Error: connect EADDRNOTAVAIL).
This is not compatible with async design. The user might perform an async operation inside their request handler, and the result of that async operation determines where they want to move the files to. When the async operation completes, the request is probably closed, and the temp files that it needed would have been auto cleaned.