Add support for updating identities on each login

It allows to automatically add new e-mail addresses when user's account is modified in LDAP. Useful especially with $config['identities_level']=1
Also, see that it is already loaded in init_ldap(). We don't want to call load_config() twice, so maybe an additional method that will load config only once, would be good. Hmm... maybe we should do this in rcube_plugin::load_config().