Refs. #10. Add commands to add and remove from the local whitelist.

 - `pyzor local_whitelist` will add to the local_whitelist, an error
    is shown if the message is already whitelisted.
 - `pyzor local_unwhitelist` will remove from the local_whitelist an
    error is show if the message is not whitelisted.

These commands can receive messages in all the different style that
are currently supported for all other methods.