Add provider to cache content

[Kaliumhexacyanoferrat]

As an operator of a web application I would like to leverage caching for specific content served by my application to improve latency and performance.

Example

var content = Resources.From(ResourceTree.FromDirectory("./"))
                       .Cached();

Host.Create()
    .Handler(content)
    .Run();

Acceptance criteria

• The functionality is implemented as a concern in the Caching module
• The implementation makes use of the caching infrastructure (see Add caching infrastructure #45)
• By default, a file-based cache is used
• The concern detects changes to cached responses to invalidate the cache
• Responses are sent with a known content length instead of Transfer-Encoding: chunked
• Acceptance tests have been added to ensure the functionality