Detect and warn about documentation with broken links.

[duncan]

There are numerous broken links in the haddock documentation on hackage. Some of these are the result of syntactic mistakes in the markup (eg forgetting to escape quotation marks) and some are just plain stale links.

We should have a mechanism to check for such problems and report them to the package author. This would probably involve parsing the html files to look for links. Detecting broken links that are internal to the docs for a single package is not too hard. Broken cross-package links is a bit harder. Detecting stale external links would need to make http GET requests.