Support cache store in reachability checks

[skalee]

No need to ping same domain all the time to check that it's reachable (see #12). Reachability tests results should be cached for performance. The cache store should be configurable and have the same interface as Rails cache. By default, there should be no cache, and the most common cache configuration should somewhat resemble following:

UriFormatValidator.cache_store = Rails.config.cache_store

For reference, Rails guide on caching.

[ronaldtse]

Agree @skalee !