Add runner.shutdown as an alternative to the exit hooks

I am trying to run luacov on the unprotected side of a sandbox, and the exit tricks aren't working since os.exit never gets called nor does the on_exit_trick.

So i just exposed on_exit as M.shutdown.  This why, I can tell luacov to save it's stats because I know when the execution is done.

Sorry for the noise in the commit.  I remove trailing white space on save and didn't realize it caused so much  noise until I was putting this PR together.  I can remove it to make it clearer if need be.