support for protocol-relative urls in external link checker
support for protocol-relative urls in external link checker

Protocol-relative urls are of the form //example.com. They get converted
to the form http://example.com before being run through the external
link checker. Of course, this assumes the protocol is http, but that is a
fairly small and safe assumption, with minimal repercussions if wrong.

hilohiro/nanoc,gjtorikian/nanoc,RubenVerborgh/nanoc,barraq/nanoc,remko/nanoc,legallybrown/nanoc,nanoc/nanoc