Reduce required args to new parser

.gitignore

@@ -0,0 +1,5 @@
+*.a
+*.exe
+*.o
+*.out
+*.so

cmd/argh-example/main.go

@@ -15,16 +15,16 @@ func main() {
 
 	log.SetFlags(0)
 
-	pt, err := argh.ParseArgs(os.Args, argh.NewParserConfig(
-		&argh.CommandConfig{
-			NValue:     argh.OneOrMoreValue,
-			ValueNames: []string{"topping"},
-			Flags: &argh.Flags{
-				Automatic: true,
-			},
+	pCfg := argh.NewParserConfig()
+	pCfg.Prog = argh.CommandConfig{
+		NValue:     argh.OneOrMoreValue,
+		ValueNames: []string{"topping"},
+		Flags: &argh.Flags{
+			Automatic: true,
 		},
-		nil,
-	))
+	}
+
+	pt, err := argh.ParseArgs(os.Args, pCfg)
 	if err != nil {
 		log.Fatal(err)
 	}

parser_config.go

@@ -7,10 +7,7 @@ const (
 )
 
 var (
-	POSIXyParserConfig = NewParserConfig(
-		nil,
-		POSIXyScannerConfig,
-	)
+	POSIXyParserConfig = NewParserConfig()
 )
 
 type NValue int
@@ -35,20 +32,24 @@ type ParserConfig struct {
 	ScannerConfig *ScannerConfig
 }
 
-func NewParserConfig(prog *CommandConfig, sCfg *ScannerConfig) *ParserConfig {
-	if sCfg == nil {
-		sCfg = POSIXyScannerConfig
-	}
+type ParserOption func(*ParserConfig)
+
+func NewParserConfig(opts ...ParserOption) *ParserConfig {
+	pCfg := &ParserConfig{}
 
-	if prog == nil {
-		prog = &CommandConfig{}
+	for _, opt := range opts {
+		if opt != nil {
+			opt(pCfg)
+		}
 	}
 
-	prog.init()
+	if pCfg.Prog.IsZero() {
+		pCfg.Prog = CommandConfig{}
+		pCfg.Prog.init()
+	}
 
-	pCfg := &ParserConfig{
-		Prog:          *prog,
-		ScannerConfig: sCfg,
+	if pCfg.ScannerConfig == nil {
+		pCfg.ScannerConfig = POSIXyScannerConfig
 	}
 
 	return pCfg
@@ -61,6 +62,13 @@ type CommandConfig struct {
 	Commands   *Commands
 }
 
+func (cCfg *CommandConfig) IsZero() bool {
+	return cCfg.NValue == NValue(0) &&
+		cCfg.ValueNames == nil &&
+		cCfg.Flags == nil &&
+		cCfg.Commands == nil
+}
+
 func (cCfg *CommandConfig) init() {
 	if cCfg.ValueNames == nil {
 		cCfg.ValueNames = []string{}