Add base URL to links

[RuedigerVoigt]

Many documents use relative links like overview.html instead of https://www.example.com/overview.html. It would be useful to convert those into absolute links before the page content is saved.

• The first step would be to find each link and to determine if it is absolute or relative. This must cover other protocols besides http and https.
• It is possible that a base-URL was set in the document code, which is not the URL of the page just crawled. This has to be found and regarded.
• To capture cases like ../../foobar.html the urllib.parse.urljoin function should be used.

The best place for this functionality seems to be an optional feature of the prettify_html function.