Add "force" option to save or update

[ralfthewise]

Often times after merging branches my "vendor" directory is left in an inconsistent state and I just want to re-vendor. However, doing a godep save or godep update ends up being a no-op because it seems like godep assumes there are no changes based on the SHAs in Godeps.json. It would be nice to be able to do godep save -f or some such that would always copy dependencies to "vendor", even if it thinks things are already up to date.

[frankcarey]

Yeah, I second this.. also seemed to happen if I manually deleted my vendor folder that it wasn't being recreated.