Improve caching

[Gummibeer]

Right now we only use a poor-man cache - so we don't load the same data during one runtime but the next runtime we will load it again.
An idea would be a cacheFor() method on the client which you can call in your own service provider and pass in a value compatible with Cache::remember() and false to don't use the Laravel cache.

As cache-key I would use something like webmentions.md5(domain) as we load all webmentions per domain and filter in PHP collections.