support scanning of a local HTML file

[afeld]

If the path to a local HTML file is given, it should be scanned, and respect absolute/relative file paths in relation to its directory. For example, when scanning /path/to/my/index.html:

• <a href="../foo.html"> references the file /path/to/foo.html
• <a href="/bar.html"> references the file /path/to/my/bar.html