Add an option to remove default tag of a project

[tukusejssirs]

When we set up default tags for projects in default_tags, we sometimes want to remove a tag or two from the list.

We might be analogous to +tag and use -tag, however, that would require -- to stop it parsing as command-line options. I don’t about the syntax (it might be an option too, like -r tag-to-remove).