If no action is specified on the command or app, print the help documentation

Rather than panic'ing or displaying an opaque error message about the
signature which is more confusing to the end user.

Fixes #562

app.go

@@ -242,6 +242,10 @@ func (a *App) Run(arguments []string) (err error) {
 		}
 	}
 
+	if a.Action == nil {
+		a.Action = helpCommand.Action
+	}
+
 	// Run default Action
 	err = HandleAction(a.Action, context)
 

app_test.go

@@ -178,6 +178,49 @@ func ExampleApp_Run_commandHelp() {
 	//    This is how we describe describeit the function
 }
 
+func ExampleApp_Run_noAction() {
+	app := App{}
+	app.Name = "greet"
+	app.Run([]string{"greet"})
+	// Output:
+	// NAME:
+	//    greet
+	//
+	// USAGE:
+	//     [global options] command [command options] [arguments...]
+	//
+	// COMMANDS:
+	//      help, h  Shows a list of commands or help for one command
+	//
+	// GLOBAL OPTIONS:
+	//    --help, -h     show help
+	//    --version, -v  print the version
+}
+
+func ExampleApp_Run_subcommandNoAction() {
+	app := App{}
+	app.Name = "greet"
+	app.Commands = []Command{
+		{
+			Name:        "describeit",
+			Aliases:     []string{"d"},
+			Usage:       "use it to see a description",
+			Description: "This is how we describe describeit the function",
+		},
+	}
+	app.Run([]string{"greet", "describeit"})
+	// Output:
+	// NAME:
+	//     describeit - use it to see a description
+	//
+	// USAGE:
+	//     describeit [arguments...]
+	//
+	// DESCRIPTION:
+	//    This is how we describe describeit the function
+
+}
+
 func ExampleApp_Run_bashComplete() {
 	// set args for examples sake
 	os.Args = []string{"greet", "--generate-bash-completion"}

command.go

@@ -187,6 +187,10 @@ func (c Command) Run(ctx *Context) (err error) {
 		}
 	}
 
+	if c.Action == nil {
+		c.Action = helpSubcommand.Action
+	}
+
 	context.Command = c
 	err = HandleAction(c.Action, context)
 

help.go

@@ -13,7 +13,7 @@ import (
 // cli.go uses text/template to render templates. You can
 // render custom help text by setting this variable.
 var AppHelpTemplate = `NAME:
-   {{.Name}} - {{.Usage}}
+   {{.Name}}{{if .Usage}} - {{.Usage}}{{end}}
 
 USAGE:
    {{if .UsageText}}{{.UsageText}}{{else}}{{.HelpName}} {{if .VisibleFlags}}[global options]{{end}}{{if .Commands}} command [command options]{{end}} {{if .ArgsUsage}}{{.ArgsUsage}}{{else}}[arguments...]{{end}}{{end}}{{if .Version}}{{if not .HideVersion}}