Allow using X509Certificate instead of Pfx

[KieranFoot]

In my attempted use of this library, I would need to pass an X509Certificate which I resolve from the local computer.
This is because it will be installed and trusted ahead of time.

[jchristn]

Were you aware of this constructor?

public Webserver(string hostname, int port, Func<HttpContext, Task> defaultRoute, X509Certificate2 sslCertificate = null)