Add support for deleting tags

[theory]

Add a -d and --delete option to the tag command to delete a tag. Refuse to remove the tag if it is used for the suffix for any reworked changes (perhaps make sure that #70 is resolved, first). I don't think we'd want a -D option to force such deletion even with such suffixes until we can rename scripts with the suffix and tell the VCS that it has been renamed (which depends on #25, of course).

[theory]

See also #22, which will want to remove deleted tags.