Shortcut key to launch in browser

This adds support for Ctrl+L to launch URL in default web browser.

Using npm module `open` to ensure it works in Win/Mac/nix and we use `keypress` module to listen for the key combination. We also update the console output so it describes what key combinations are accepted:

    Serving files from /path/to/files at http://localhost:8000
    Press Ctrl+L to launch in browser
    Press Ctrl+C to quit