Allow users to write in the email after entering "ipatool auth login" instead of passing it in as an argument

[YousufSSyed]

Description

Right now you have to pass --email email@icloud.com for the command to work. If the user doesn't, instead it could just ask after the fact instead:

> ipatool auth login
> Enter email: email@icloud.com

Users would also be less likely to enter it in incorrectly if they focus on just writing in the email, and not also writing in the whole command at the same time. And especially if they have a small terminal window width.