Make `--help`/`--version` explicit aliases for `help`/`version`

Before, the argument parser would translate `--help` and `--version` to
their equivalents without the dashes. Now it's not the parser's
responsibility to do that anymore. Instead, wire them up as subcommand
aliases in the runner.

commands/help.go

@@ -17,7 +17,7 @@ var cmdHelp = &Command{
 func init() {
 	cmdHelp.Run = runHelp
 
-	CmdRunner.Use(cmdHelp)
+	CmdRunner.Use(cmdHelp, "--help")
 }
 
 func runHelp(cmd *Command, args *Args) {

commands/runner.go

@@ -54,8 +54,11 @@ func (r *Runner) All() map[string]*Command {
 	return r.commands
 }
 
-func (r *Runner) Use(command *Command) {
+func (r *Runner) Use(command *Command, aliases ...string) {
 	r.commands[command.Name()] = command
+	if len(aliases) > 0 {
+		r.commands[aliases[0]] = command
+	}
 }
 
 func (r *Runner) Lookup(name string) *Command {

commands/version.go

@@ -19,7 +19,7 @@ var cmdVersion = &Command{
 }
 
 func init() {
-	CmdRunner.Use(cmdVersion)
+	CmdRunner.Use(cmdVersion, "--version")
 }
 
 func runVersion(cmd *Command, args *Args) {