squash merge from dev branch

vendor/gopkg.in/alecthomas/kingpin.v2/actions.go

@@ -0,0 +1,42 @@
+package kingpin
+
+// Action callback executed at various stages after all values are populated.
+// The application, commands, arguments and flags all have corresponding
+// actions.
+type Action func(*ParseContext) error
+
+type actionMixin struct {
+	actions    []Action
+	preActions []Action
+}
+
+type actionApplier interface {
+	applyActions(*ParseContext) error
+	applyPreActions(*ParseContext) error
+}
+
+func (a *actionMixin) addAction(action Action) {
+	a.actions = append(a.actions, action)
+}
+
+func (a *actionMixin) addPreAction(action Action) {
+	a.preActions = append(a.preActions, action)
+}
+
+func (a *actionMixin) applyActions(context *ParseContext) error {
+	for _, action := range a.actions {
+		if err := action(context); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (a *actionMixin) applyPreActions(context *ParseContext) error {
+	for _, preAction := range a.preActions {
+		if err := preAction(context); err != nil {
+			return err
+		}
+	}
+	return nil
+}