Restructure connect to connect_with_prefs or something

TorClient::connect takes an Option<ConnectPrefs>, as a way to provide both a convenience function that doesn't take preferences, and a full function which does. This is not very Rustic.

This (and the other methods that take an Option<ConnectPrefs>) should be refactored. Perhaps connect_with_prefs, or a .with_prefs(). Perhaps there should be a way to alter the default prefs for a particular TorClient.