`base_url/1` needs default setting

[dcnorris]

One important, typical use-case for doclog will be preparing Scryer PR's in a forked repo. I have found that, in order to build documentation that I can view in browser with the nice styling, I needed to modify the doclog.config.pl included in scryer-prolog root as follows:

 copy_file("logo/scryer.png", "scryer.png").
 copy_file("learn/Spanish_deck_Fournier.jpg", "learn/Spanish_deck_Fournier.jpg").
 copy_file("learn/brisca-interactive.png", "learn/brisca-interactive.png").
+base_url("/home/david/scryer-prolog/doclog/output/").

Ideally, this would work out-of-the-box. Is there some way to arrange that? (If this should be a Scryer Issue or Discussion, let me know and I'll post there.)