allow configuration of multipart cutoff

Currently it's possible that a very large multi-file upload can run a server out of available file handles. 

I realize that setting a specific cut off for everybody is too restricting, but I think it'd be very useful to set a configurable point after which rack will stop handling new parts (and thus creating new tempfiles).

/cc @camilo, @arthurnn, @bslobodin
Because exploiting semi-colons to do one-liner is lame? 

But it look like the `class SubClass < ParentClass; end` way is already used in that file, so for consistency I'll go that way.