squash merge from dev branch

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

@@ -0,0 +1,33 @@
+package kingpin
+
+// HintAction is a function type who is expected to return a slice of possible
+// command line arguments.
+type HintAction func() []string
+type completionsMixin struct {
+	hintActions        []HintAction
+	builtinHintActions []HintAction
+}
+
+func (a *completionsMixin) addHintAction(action HintAction) {
+	a.hintActions = append(a.hintActions, action)
+}
+
+// Allow adding of HintActions which are added internally, ie, EnumVar
+func (a *completionsMixin) addHintActionBuiltin(action HintAction) {
+	a.builtinHintActions = append(a.builtinHintActions, action)
+}
+
+func (a *completionsMixin) resolveCompletions() []string {
+	var hints []string
+
+	options := a.builtinHintActions
+	if len(a.hintActions) > 0 {
+		// User specified their own hintActions. Use those instead.
+		options = a.hintActions
+	}
+
+	for _, hintAction := range options {
+		hints = append(hints, hintAction()...)
+	}
+	return hints
+}