feat(label): address labels by name and add import and clone

label edit bug and label delete kind/feature failed with "invalid resource
number", and label create took its name as --name and required
--color. Like gh, label create takes the name as an argument and picks
a random color without -c; label edit and delete take names or IDs
(delete several at once, after a confirmation that names them), and
-c/-d/-n/-f are the gh short flags. Without a name on a terminal, label
create asks for the name, color and description.

label list shows ID NAME COLOR DESCRIPTION FLAGS with a colored swatch
and exclusive and archived labels flagged. Mutations report ✓ lines.
label import reads a label file or stdin (label create --file still
works), label clone copies another repository's labels, skipping those
that exist unless --force, and label export writes to stdout by default
or to -O/--output.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/validation_test.go

@@ -71,7 +71,7 @@ func TestUsageErrorsPrecedeRequests(t *testing.T) {
 		{[]string{"issue", "list", "--page", "0"}, `"--page" flag: must be a positive integer`},
 		{[]string{"issue", "list", "--label", "bug, "}, "label names cannot be empty"},
 		{[]string{"label", "create", "--name", "bug", "--color", "wrong"}, "--color requires six hexadecimal digits"},
-		{[]string{"label", "edit", "1"}, "supply label fields to edit"},
+		{[]string{"label", "edit", "1"}, "nothing to edit; pass a flag such as --title (see --help)"},
 		{[]string{"label", "list", "--org", "team", "-R", "a/b"}, "--org and --repo are mutually exclusive"},
 		{[]string{"milestone", "close", " ", "--by-name"}, "milestone title or ID cannot be empty"},
 		{[]string{"milestone", "create", "--title", "v1", "--deadline", "invalid"}, "invalid --deadline"},