Support asynchronous URL reachability validations

[skalee]

As pointed out by @ronaldtse in #12 (comment), reachability validation may be slow because of external HTTP requests. Some backgrounding solution is desired. Either as an implemented feature or at least as documentation entry, example, or something like that. Probably the latter.

[ronaldtse]

Agree that documentation is the better idea. We need a working example to show.