Allow passing a username directly

[GaetanLepage]

Hello ! This is a proposal to add a new option --user (or --default-user) which would make possible to call tuigreet with a username to be pre-filled. Of course, someone could already user the --remember but it could be useful to be able to 'hardcode' directly the desired username in the CLI arguments.

Note: I have almost 0 knowledge in Rust. The provided code has to be taken with care !