Make process.exit optional

Any interest in making the automatic process exiting an optional thing that happens?  By default the behavior is the same, but there's an option to disable the process.exit() call and you become responsible for making your tests clean up and exit node properly?  A lot of the testing I do is with database stuff, and it's important to make __sure__ nothing is keeping the event loop alive after my tests run.  I'd happily send a pull request over if you're open to it.