support scanning of a local directory

[afeld]

Depends on #6.

If the path to a local HTML file is given, it should be scanned. If the path to a local directory is given, it should

• Scan all HTML files therein, or
• Scan recursively from the index.html (depends on recursively scan all pages under the given path #4), and report any orphaned pages

Not sure which is preferable. Either case would be helpful for checking the output of static site generators.