Display UsageText on Commands and Subcommands

If it is set, otherwise build the usage.

Fixes #592

help.go

@@ -47,7 +47,7 @@ var CommandHelpTemplate = `NAME:
    {{.HelpName}} - {{.Usage}}
 
 USAGE:
-   {{.HelpName}}{{if .VisibleFlags}} [command options]{{end}} {{if .ArgsUsage}}{{.ArgsUsage}}{{else}}[arguments...]{{end}}{{if .Category}}
+   {{if .UsageText}}{{.UsageText}}{{else}}{{.HelpName}}{{if .VisibleFlags}} [command options]{{end}} {{if .ArgsUsage}}{{.ArgsUsage}}{{else}}[arguments...]{{end}}{{end}}{{if .Category}}
 
 CATEGORY:
    {{.Category}}{{end}}{{if .Description}}
@@ -67,7 +67,7 @@ var SubcommandHelpTemplate = `NAME:
    {{.HelpName}} - {{if .Description}}{{.Description}}{{else}}{{.Usage}}{{end}}
 
 USAGE:
-   {{.HelpName}} command{{if .VisibleFlags}} [command options]{{end}} {{if .ArgsUsage}}{{.ArgsUsage}}{{else}}[arguments...]{{end}}
+   {{if .UsageText}}{{.UsageText}}{{else}}{{.HelpName}} command{{if .VisibleFlags}} [command options]{{end}} {{if .ArgsUsage}}{{.ArgsUsage}}{{else}}[arguments...]{{end}}{{end}}
 
 COMMANDS:{{range .VisibleCategories}}{{if .Name}}
    {{.Name}}:{{end}}{{range .VisibleCommands}}

help_test.go

@@ -283,7 +283,6 @@ EXAMPLES:
 			},
 		},
 	}
-
 	output := &bytes.Buffer{}
 	app.Writer = output
 	app.Run([]string{"foo", "help", "frobbly"})
@@ -301,6 +300,50 @@ EXAMPLES:
 	}
 }
 
+func TestShowSubcommandHelp_CommandUsageText(t *testing.T) {
+	app := &App{
+		Commands: []Command{
+			{
+				Name:      "frobbly",
+				UsageText: "this is usage text",
+			},
+		},
+	}
+
+	output := &bytes.Buffer{}
+	app.Writer = output
+
+	app.Run([]string{"foo", "frobbly", "--help"})
+
+	if !strings.Contains(output.String(), "this is usage text") {
+		t.Errorf("expected output to include usage text; got: %q", output.String())
+	}
+}
+
+func TestShowSubcommandHelp_SubcommandUsageText(t *testing.T) {
+	app := &App{
+		Commands: []Command{
+			{
+				Name: "frobbly",
+				Subcommands: []Command{
+					{
+						Name:      "bobbly",
+						UsageText: "this is usage text",
+					},
+				},
+			},
+		},
+	}
+
+	output := &bytes.Buffer{}
+	app.Writer = output
+	app.Run([]string{"foo", "frobbly", "bobbly", "--help"})
+
+	if !strings.Contains(output.String(), "this is usage text") {
+		t.Errorf("expected output to include usage text; got: %q", output.String())
+	}
+}
+
 func TestShowAppHelp_HiddenCommand(t *testing.T) {
 	app := &App{
 		Commands: []Command{