Feature: Support resolving from default hosts file
(#75, #76 and #77 by @clue)

This means that resolving hosts such as localhost will now work as
expected across all platforms with no changes required:

$resolver->resolve('localhost')->then(function ($ip) {
    echo 'IP: ' . $ip;
});

The new HostsExecutor exists for advanced usage and is otherwise used
internally for this feature.