Add HideHelpCommand

While `HideHelp` hides both `help` command and `--help` flag, `HideHelpCommand`
only hides `help` command and leave `--help` flag as-is.

The behavior of `HideHelp` is untouched in this commit.

Fix #523
Replace #636

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

command.go

@@ -41,8 +41,11 @@ type Command struct {
 	Flags []Flag
 	// Treat all flags as normal arguments if true
 	SkipFlagParsing bool
-	// Boolean to hide built-in help command
+	// Boolean to hide built-in help command and help flag
 	HideHelp bool
+	// Boolean to hide built-in help command but keep help flag
+	// Ignored if HideHelp is true.
+	HideHelpCommand bool
 	// Boolean to hide this command from help or completion
 	Hidden bool
 	// Boolean to enable short-option handling so user can combine several
@@ -236,6 +239,7 @@ func (c *Command) startApp(ctx *Context) error {
 	app.Commands = c.Subcommands
 	app.Flags = c.Flags
 	app.HideHelp = c.HideHelp
+	app.HideHelpCommand = c.HideHelpCommand
 
 	app.Version = ctx.App.Version
 	app.HideVersion = ctx.App.HideVersion