Add a last-ditch attempt to see if there's an md file on the end of a URL

[davep]

Currently Frogmouth makes a pretty simple check to see if a file at the end of a URL is a Markdown file; quite simply by looking at the name of the file and seeing if its extension is one of the "standard" extensions. The advantage of this is that it's going to work in almost all situations where someone's wanting to view a remote markdown file, and it's not an expensive decision to make (no call out to a remote resource is needed before deciding to try).

As #77 has demonstrated though, there's some merit, albeit likely less-frequently needed, in diving a little deeper. I don't think we should dive deeper every time we're looking at a URL; but I think there's no harm in Frogmouth, once it's decided to just open the URL up in the browser, checking if there's a redirect (with a HEAD request) and having one final glance at where the request will end up.

Doing it this way will have the benefit of only making the "expensive" decision once all other routes to loading the file have failed.