Fix error when printing usage for multi-value arguments

We try to compare []strings, which are uncomparable types:

`panic: runtime error: comparing uncomparable type []string`

1 files changed, 1 insertions, 1 deletions

diff --git a/usage.go b/usage.go
index fa01229..23f7aa5 100644
--- a/usage.go
+++ b/usage.go
@@ -107,7 +107,7 @@ func printOption(w io.Writer, spec *spec) {
 	v := spec.dest
 	if v.IsValid() {
 		z := reflect.Zero(v.Type())
-		if v.Interface() != z.Interface() {
+		if v.Type().Comparable() && z.Type().Comparable() && v.Interface() != z.Interface() {
 			fmt.Fprintf(w, " [default: %v]", v)
 		}
 	}