feat: stdin default

Signed-off-by: jolheiser <john.olheiser@gmail.com>

cmd/cfg/main.go

@@ -1,9 +1,9 @@
 package main
 
 import (
-	"errors"
 	"flag"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -40,21 +40,29 @@ func maine() error {
 		return err
 	}
 
-	if fs.NArg() < 1 {
-		return errors.New("cfg requires at least an input file")
-	}
-
 	cleanArg := func(arg string) string {
 		return strings.TrimPrefix(filepath.Ext(arg), ".")
 	}
 
-	in := fs.Arg(0)
+	in := os.Stdin
+	if fs.NArg() > 0 {
+		fi, err := os.Open(fs.Arg(0))
+		if err != nil {
+			return err
+		}
+		defer fi.Close()
+		in = fi
+
 		if unmarshal == nil {
 			if err := fromFunc(cleanArg(fs.Arg(0))); err != nil {
 				return err
 			}
 		}
 
+	} else if unmarshal == nil {
+		unmarshal = cfg.JSON.Unmarshal
+	}
+
 	out := os.Stdout
 	if fs.NArg() > 1 {
 		fi, err := os.Create(fs.Arg(1))
@@ -75,7 +83,7 @@ func maine() error {
 
 	var data any
 
-	inData, err := os.ReadFile(in)
+	inData, err := io.ReadAll(in)
 	if err != nil {
 		return err
 	}