Add hook support.

help.go

@@ -1,7 +1,6 @@
 package kong
 
 import (
-	"io"
 	"text/template"
 )
 
@@ -18,20 +17,20 @@ usage: {{.Name}}
 
 var defaultHelpTemplate = template.Must(template.New("help").Parse(defaultHelp))
 
-// WriteHelp to w.
-//
-// If w is nil, the default stdout writer will be used.
-//
-// If args are provided, help will be written in the context o
-func (k *Kong) WriteHelp(w io.Writer, args ...interface{}) error {
-	if w == nil {
-		w = k.stdout
+// Help returns a Hook that will display help and exit.
+func Help(tmpl *template.Template, tmplctx map[string]interface{}) Hook {
+	return func(app *Kong, ctx *Context, trace *Trace) error {
+		merged := map[string]interface{}{
+			"Application": app.Model,
+		}
+		for k, v := range tmplctx {
+			merged[k] = v
+		}
+		err := tmpl.Execute(app.Stdout, merged)
+		if err != nil {
+			return err
+		}
+		app.Exit(0)
+		return nil
 	}
-	ctx := map[string]interface{}{
-		"Application": k.Model,
-	}
-	for k, v := range k.helpContext {
-		ctx[k] = v
-	}
-	return k.help.Execute(w, ctx)
 }