Add a warn-test flag to update

[magik6k]

This is the less safe way to approach https://github.com/ipfs/gx-workspace/issues/8, I don't call it perfect, but it get's the work done

[Stebalien]

This is a step in the right direction (although I'd still prefer a "rollback on failure" option or a "Continue [y/N]?" option).

[whyrusleeping]

Yeah, lets go with a prompt allowing the user to select whether or not they want to continue