tag: Disallow adding malformed tags to messages

tag: Disallow adding malformed tags to messages

This disallows adding empty tags, since nothing but confusion follows
in their wake, and disallows adding tags that begin with "-" because
they are also confusing, the tag "-" is impossible to remove using the
CLI, and because the syntax for removing such tags conflicts with long
argument syntax.

This does not place any restrictions on what tags can be removed, as
that would make it difficult for people who have the misfortune of
already having malformed tags to remove these tags.