Establish session with a separate command

[jliuhtonen]

When there is no session, blu-hawaii currently starts in interactive mode prompting the user to approve the API client in Last.fm. As the software is usually run in non-interactive mode, it can be confusing to prompt for input.

Suggestion: Add a separate parameter to start login in interactive mode like "blu-hawaii --authenticate" and just fail the main flow with non-zero exit code if there is no session.