Hack to use multiple instances of optimist.

We need to call optimist for each operation in a given pipeline. It is designed to be called once per script, so variables are not reset between calls, and lots of stale information appears in the returned argv object.

I don’t know if deleting from require.cache is ok, though…

I found this solution [on stackoverflow](http://stackoverflow.com/a/11477602). If there’s a better one, please let me know / apply it!