Add ability to remove a tag from all repos

[timabell]

Can just just make the repo folder arg optional

gitopolis tag --remove some-tag some-repo  # remove from one
gitopolis tag --remove some-tag # remove from all

[timabell]

workaround: list repos for a tag and pipe through xargs into tag remove command:

gitopolis list -t some-tag | xargs gitopolis tag --remove some-tag

[timabell]

if we implement this it's important to make it a bit easier to undo as it's destructive.

couple of ideas

• add a .bak copy of config
• spit out which repos were affected in the terminal - could even give an undo example to re-add them