Extending authentication methods

Currently `standard` is the only authentication mechanism supported. I would like to add more. For example, HMAC-SHA1 can be computed on a Yubikey.

There are couple of ways this could be done:

- Implement the new authentication method in upstream and add a branch to the if-else tree in configuration parsing to instantiate it when specified.

- Add a global or config-level authentication method registry, such that the caller could do something like `AddAuthenticationProvider(name string, provider auth.Provider)` before configuration is instantiated. Then configuration parsing would know what this name means.

Is the project interested in a PR for either of these?