[feature]allow users to specify the passwords for different blog sites

Hey Bowen,
First of all, thank you for your great octopress plugin. It saves me a lot of time XD
I have noticed that the latest usage assumes all the blogs using the same password, which is not my case. So I add a feature to allow the "PASSWD" argument left empty, and then users can specify their password site by site.
Although I have checked my code with some testcases, further testing might still be required since I am a newbie to ruby~