Add --init-completion flag to print completion code

This flag takes as input 'bash' or 'zsh' and generates the completion
code for the specified shell.

app.go

@@ -138,6 +138,7 @@ func (a *App) Setup() {
 
 	if a.EnableBashCompletion {
 		a.appendFlag(BashCompletionFlag)
+		a.appendFlag(InitCompletionFlag)
 	}
 
 	if !a.HideVersion {
@@ -176,6 +177,11 @@ func (a *App) Run(arguments []string) (err error) {
 		return nil
 	}
 
+	var done bool
+	if done, err = checkInitCompletion(context); done {
+		return nil
+	}
+
 	if err != nil {
 		if a.OnUsageError != nil {
 			err := a.OnUsageError(context, err, false)