Add Twitter Options: forceLogin and requestPermissions

This PR preserves backwards compatibility. It adds two options to the Twitter package. When requesting a credential, there are now two options.

The forceLogin option means force_login gets passed through to Twitter, telling it to present the authentication form as a login form, disregarding any user currently signed in to Twitter.com at the time.

The requestPermissions option allows an app to specify a lower permission level if it doesn't want use the one the app is configured with at dev.twitter.com. Valid options are 'read' and 'write'.

Closes #616, #1564