Add support for dependent pages

Instead of returning a string of html, you return a map:

 {:contents "the html",
  :dependent-pages {"/uri" "contents"}}

These dependent pages are then also served and exported.

This allows you to keep your page-creation logic sane while
creating subpages for them, especially when using lazy evaluation of
pages with the get-page feature.
Yeah, that sounds ok. I try to avoid booleans as arguments, but I guess it's the least awkward choice in this case.