Adds a new hook to tweak command description at runtime

For one feature I'm currently implementing I need to be able to tweak a command description based on some runtime information.
My main use case is to customize the description (basically rendering it as a template but with some context values that we can't provide generically).
So I ended up with this solution.

The idea is to allow one to register a hook that will be called during command help rendering to compute the command description.

command.go

@@ -44,6 +44,8 @@ type Command struct {
 	Usage string
 	// A longer explanation of how the command works
 	Description string
+	// or a function responsible to render the description
+	DescriptionFunc DescriptionFunc
 	// The category the command is part of
 	Category string
 	// An action to execute before any sub-subcommands are run, but after the context is ready

funcs.go

@@ -33,6 +33,10 @@ type ActionFunc func(*Context) error
 // CommandNotFoundFunc is executed if the proper command cannot be found
 type CommandNotFoundFunc func(*Context, string) error
 
+// DescriptionFunc is used by the help generation to display a description when
+// its computation is intensive or needs runtime information
+type DescriptionFunc func(*Command, *Application) string
+
 // FlagStringFunc is used by the help generation to display a flag, which is
 // expected to be a single line.
 type FlagStringFunc func(Flag) string

help.go

@@ -149,6 +149,10 @@ func ShowAppHelp(c *Context) error {
 // ShowCommandHelp prints help for the given command
 func ShowCommandHelp(ctx *Context, command string) error {
 	if c := ctx.App.Command(command); c != nil {
+		if c.DescriptionFunc != nil {
+			c.Description = c.DescriptionFunc(c, ctx.App)
+		}
+
 		HelpPrinter(ctx.App.Writer, CommandHelpTemplate, c)
 		return nil
 	}

help_test.go

@@ -237,6 +237,31 @@ func TestShowCommandHelp_CommandAliases(t *testing.T) {
 	}
 }
 
+func TestShowCommandHelp_DescriptionFunc(t *testing.T) {
+	app := &Application{
+		Commands: []*Command{
+			{
+				Name:        "frobbly",
+				Description: "this is not my custom description",
+				DescriptionFunc: func(*Command, *Application) string {
+					return "this is my custom description"
+				},
+				Action: func(ctx *Context) error {
+					return nil
+				},
+			},
+		},
+	}
+
+	output := &bytes.Buffer{}
+	app.Writer = output
+	app.Run([]string{"foo", "help", "frobbly"})
+
+	if !strings.Contains(output.String(), "this is my custom description") {
+		t.Errorf("expected output to include result of DescriptionFunc; got: %q", output.String())
+	}
+}
+
 func TestShowAppHelp_HiddenCommand(t *testing.T) {
 	app := &Application{
 		Commands: []*Command{