FR: Ability to always call next()

This was brought up in https://github.com/expressjs/serve-static/issues/68#issuecomment-231559148 but I was wondering if this could be revisited. We may want to have logging/other tasks done per request but not block on sending a response. I also found that res.on("finish") doesn't fire if a client disconnects mid-request.

Something like the fallthrough option that fires next() on the "end" event of the underlying stream.