Add commands to "login" and "logout" twitter accounts from in BarnOwl

[nelhage@mit.edu]

We should have commands to manipulate the state of Twitter accounts without editing the config and reloading. I'm envisioning something like the following features:

• The ability to :twitter-login without a config file at all
• :twitter-login and :twitter-logout that can control whether an account is currently considered "active"
• The ability to mark an account as !autologin, so that it has to be manually logged in.
• The ability to store an account in .owl/twitter without a password, so you can store config, but have to log in manually with a password.
• Anything else that makes sense.

This should be relatively easy, but requires a bit of design thought.