added username to password entries

I created a new formatter that supports usernames in password entries. When creating an entry, the user is now prompted for a username and a password. The username is optional.
When calling 'get', the user sees the username in clear and the password is copied in the clipboard as usual.
This is particularly helpful if you have accounts where you can't use the same username (e.g. your email address) or - worse - when username and password are fixed by the system (had this experience personally with some institutional websites).