Add instance command registry and composition

registry.go

@@ -0,0 +1,27 @@
+package commandkit
+
+import "fmt"
+
+// Register adds a batch of root commands to the application. The batch is
+// validated before the registry is changed.
+func (a *App) Register(commands ...Command) error {
+	if a == nil {
+		return fmt.Errorf("cannot register commands on a nil app")
+	}
+
+	updated := make(map[string]*commandNode, len(a.commands)+len(commands))
+	for name, node := range a.commands {
+		updated[name] = node
+	}
+	for _, command := range commands {
+		if err := validateCommandDeclaration(command); err != nil {
+			return err
+		}
+		if _, exists := updated[command.name]; exists {
+			return fmt.Errorf("command path %q is already registered", command.name)
+		}
+		updated[command.name] = &commandNode{command: cloneCommand(command)}
+	}
+	a.commands = updated
+	return nil
+}