Option for hiding username/password fields.
 - Add options for hiding either or both of the username
   and password input fields.
 - Remove the option to disable/enable the MOTD, because we need
   it to attach to the submit button if both username and password
   fields are disabled.  If users don't want a MOTD, they can just
   leave it blank.
 - Resolves #219
 - This should make it easier for plugin developers who want to support
   things like smart cards, biometrics, etc.
 - Configuration options placed under "Credential Provider Options" tab.