feat: add stdin support for piped input

- Allow reading markdown from stdin when no file argument provided
- Support explicit '-' argument to read from stdin
- Update README with stdin usage examples
- Update command usage to reflect optional file argument

Author: codehakase

.gitignore

@@ -0,0 +1 @@
+md

README.md

@@ -28,17 +28,28 @@ go build -o md .
 
 ```
 Usage:
-  md [flags] <markdown-file>
+  md [flags] [markdown-file]
 
 Flags:
-  -h, --help   help for md
-  -v, --vim    Enable vim-style navigation
+  -h, --help    help for md
+  -p, --plain   Render entire markdown to stdout (no pager)
 ```
 
+By default, `md` displays content in a vim-style pager. Use `--plain` to output directly to stdout.
+
+### Reading from stdin
+
+You can pipe markdown content directly to `md`:
+
+```bash
+cat README.md | md
+echo "# Hello World" | md
+curl -s https://example.com/doc.md | md
+```
 
 ### Vim Navigation Keys
 
-When using `--vim` mode, you can navigate using:
+In the default pager mode, you can navigate using:
 
 - `j` / `k` - Move down/up
 - `gg` - Go to top

main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 
@@ -15,49 +16,60 @@ import (
 
 var (
 	plainMode bool
-	watchMode bool
 )
 
 var rootCmd = &cobra.Command{
-	Use:   "md [flags] <markdown-file>",
+	Use:   "md [flags] [markdown-file]",
 	Short: "A markdown renderer and viewer for the terminal",
 	Long: `md is a command-line tool that renders markdown files with syntax highlighting
-and provides options for vim-style navigation.`,
-	Args: cobra.ExactArgs(1),
-	RunE: func(cmd *cobra.Command, args []string) error {
-		filename := args[0]
-		if !filepath.IsAbs(filename) {
-			var err error
-			filename, err = filepath.Abs(filename)
-			if err != nil {
-				return fmt.Errorf("error resolving file path: %v", err)
-			}
-		}
-
-		if _, err := os.Stat(filename); os.IsNotExist(err) {
-			return fmt.Errorf("file not found: %s", filename)
-		}
+and provides options for vim-style navigation.
 
+If no file is provided, md reads from stdin.`,
+	Args: cobra.MaximumNArgs(1),
+	RunE: func(cmd *cobra.Command, args []string) error {
 		themeManager := theme.New()
 		mdRenderer := renderer.New(themeManager)
 		codeHighlighter := highlighter.New(themeManager)
 		mdViewer := viewer.New()
 
-		renderAndDisplay := func() error {
-			content, err := mdRenderer.RenderFile(filename, codeHighlighter)
-			if err != nil {
-				return fmt.Errorf("rendering error: %v", err)
+		var content string
+		var err error
+
+		if len(args) == 0 || args[0] == "-" {
+			// Read from stdin
+			stdinContent, readErr := io.ReadAll(os.Stdin)
+			if readErr != nil {
+				return fmt.Errorf("error reading from stdin: %v", readErr)
+			}
+			if len(stdinContent) == 0 {
+				return fmt.Errorf("no input provided")
+			}
+			content, err = mdRenderer.RenderContent(stdinContent, codeHighlighter)
+		} else {
+			filename := args[0]
+			if !filepath.IsAbs(filename) {
+				filename, err = filepath.Abs(filename)
+				if err != nil {
+					return fmt.Errorf("error resolving file path: %v", err)
+				}
 			}
 
-			if plainMode {
-				fmt.Print(content)
-				return nil
-			} else {
-				return mdViewer.DisplayInVimMode(content)
+			if _, statErr := os.Stat(filename); os.IsNotExist(statErr) {
+				return fmt.Errorf("file not found: %s", filename)
 			}
+
+			content, err = mdRenderer.RenderFile(filename, codeHighlighter)
 		}
 
-		return renderAndDisplay()
+		if err != nil {
+			return fmt.Errorf("rendering error: %v", err)
+		}
+
+		if plainMode {
+			fmt.Print(content)
+			return nil
+		}
+		return mdViewer.DisplayInVimMode(content)
 	},
 }