Add a detached mode

[rasbt]

🚀 Feature

It would be nice to add a --detach mode similar to jekyll serve to detach the session.

Motivation

This could be useful for testing purposes and usability etc so it doesn't block the main terminal.

Pitch

The equivalent of

jekyll serve --detach
...
      Generating... 
                    done in 3.982 seconds.
 Auto-regeneration: disabled when running server detached.
    Server address: http://127.0.0.1:4000/
Server detached with pid '76023'. Run `pkill -f jekyll' or `kill -9 76023' to stop the server.

Alternatives

Additional context

[github-actions]

Hi! thanks for your contribution!, great first issue!

[williamFalcon]

this would require introducing a CLI which may not be a bad idea but idk if it’s needed? we all want to keep litserve simple but maybe CLI can still keep it simple