Add option to accept new fingerprint #3

Right now, this project will complain when you're connecting to a server for the first time and you have not already accepted have its fingerprint. You may simply connect on the command line with ssh user@example.com and accept this initial fingerprint (TOFU: trust on first use).

We should add an option to pass an explicit fingerprint to this library so that one no longer has to connect to a server on the command line before being able to use this library.