yubikey - support internal password server

Please expose the Yubikey authentication server URL so that it can be pointed to a internal auth server. Our Yubikeys have a custom key so they can't be used on the public Yubikey server. Also please add a optional field that can be concatenated into the REST URL for authfile, so a complete url would be something like:

https://internalserver.company.com/wsapi/2.0/verify?id=1&authfile=/some/file/path&nonce=somethingrandom&otp=Full_Yubikey_OTP

Implementation could be as simple as concatenating base URL up to verify, followed by your usual parameters and the concatenating "&" and the additional parameters field.

If these are null it should insert hidden default parameters.