CLI: make login easier for use in scripts

[wonder-sk]

Right now to use the CLI in a script, one needs to do something like this to set up the environment:

mergin login --login myUsername --password myPassword | tail -2 > env_vars
source env_vars

I would suggest that

1. other commands recognize MERGIN_URL, MERGIN_USERNAME, MERGIN_PASSWORD env variables - so instead of requiring an existing token, if it is not present, commands would log in using the provided username/password. This would avoid the need to run 'mergin login' altogether.
2. 'mergin login' should not output "Mergin URL: " + c.url so that a script does not need to do "tail -2" to remove it. The URL is in exported MERGIN_URL variable anyway...