fix(auth): add support for setting auth header and cookie manually

[ryeguard]

An approach for mitigating issue #113.
This PR allows the user to set the Authorization header and JWT_FGP cookie through command line args, e.g.,

export TOKEN="longer_token_here"
export JWT_FGP="shorter-cookie-here"
garmin-backup --token=$TOKEN --jwt_fgp=$JWT_FGP username

Where the token/cookie can be found under the Network>Headers/Cookies tab in your browser's DevTools in requests to connect.garmin.com.

[atomical]

This is a cool idea. This project doesn't work without the captcha, right?

[tymmej]

It looks like it solves #108 also.