Make various improvements and fixes.

Depagination:
* Activate only when new CLI option `-p` is given.
* Remove several lines of dead code.
* Restrict the list of possible links to the next page
  such that the URLs must contain as their protocol field
  either http or https.

Full HTML document (i.e. non-fragment) output:
* Include a meta-tag encoding declaration.
* Include the shortened title in html>head>title.
Fix the URL filtering for the depaginator.

Previously, any URLs that were not protocol-absolute
would be rejected. Now, only the trivial pathology of
the "javascript" URL protocol is rejected.

Note that other pathological cases may exist.