Add `--untag` flag to `remove` command

[bmicklea]

Describe the problem you're trying to solve There's no way to remove a tag from a ModelKit if it only has one tag (you end up removing the ModelKit).

Describe the solution you'd like A flag to remove a tag (even the last tag) would help.

Describe alternatives you've considered Adding the new tag and then removing the old. This might be better if we think we always want a tag for every ModelKit...