Added a simple password prompt to avoid storing passwords in plain text

Currently, the password of the user must be stored in the `~/.dl.rc` file in plaintext. While I understand that this can be useful in order to automate tasks easily, it really isn't very secure at all.

I've added a simple check in - if the password is not supplied as part of the config file, `getpass` is used to prompt the user for their password. This remains backwards compatible with the current way of doing things, but allows a user to avoid having to hard code their password in the config file.