Add -output flag to write the graph to a file

[qwerty8811]

Summary
Added an -output flag to write dependency graph output (DOT or Mermaid) to a file instead of stdout. Provides a documented graph → path option (visible in -help) without relying on shell redirects, and surfaces file I/O errors directly from the tool.

What changed

• New flag: -output - file path for graph output (defaults to stdout if empty).
• Refactor: graphvizPrinter and mermaidPrinter now accept an io.Writer and write via fmt.Fprint instead of stdout.

Usage

godepgraph -output deps.dot ./your/package
godepgraph -format mermaid -output deps.mmd ./your/package