Move command init into cmd_* files

cmd/khepri/cmd_cat.go

@@ -10,6 +10,10 @@ import (
 	"github.com/fd0/khepri/backend"
 )
 
+func init() {
+	commands["cat"] = commandCat
+}
+
 func commandCat(be backend.Server, key *khepri.Key, args []string) error {
 	if len(args) != 2 {
 		return errors.New("usage: cat [blob|tree|snapshot|key|lock] ID")