Adding in on-update variable, for those who want to re-run on update

Recently, I arrived at a problem where I needed to re-download a file every time I ran a buildout update (the file updates every single time, so I needed a non-cached fresh version). Seemed like the easier way to do this was follow the buildout convention of an on_update flag, which allows one to re-run directives on the update step if desired.

I'd be happy to make changes if you think there's a different way you would like this done. :)

Thanks!