Support premium access

[janis-ax]

Client should support premium access.

It should be possible by adding the following lines into cli.py

 p.add_argument("-U", "--username", env_var="USERNAME",
                   help="username for the languagetool premium API"
                   )
    p.add_argument("-P", "--api-key", env_var="API_KEY", help="apiKey for the languagetool premium API",)