update to allow better username/password discovery

- update 'username' command to return a list of results if searching by
  domain returns multiple hits
- update 'username' and 'password' commands to accept 1 or more
  arguments which will be searched sequentially until a result (or
  results in the case of 'username') is found. this will allows scripts
  to supply multiple possible keys in the case that the user supplied
  arg is ambiguous (like foo@bar.com: is that just the username or is
  that the username at domain?).
improve handling of arg supplied to vimperator keyring commands

- allow the login command to accept just the domain name to search for
  both the username and password (eg: keyring gmail.com)
- allow the password command to accept just the username, saving the
  user from typing the domain and preventing accidentally supplying
  login credentials to a site spoofying a legit site's domain.