append logs if they exist, otherwise create

[Freyert]

Small change, but I hope it helps people improve lemurs and debug more easily. For some reason I've never been able to log back in after a hibernate, but I could never debug it because the logs kept on getting overwritten on restart.

This change opens the logs in append mode, but keeps the create if non-existant as well.

https://doc.rust-lang.org/std/fs/struct.OpenOptions.html

[Freyert]

The clippy errors are not related to my change :)

I don't feel particularly devoted to this change and I'm bad at responding so if you want to change or tweak it please feel free. Otherwise just merge it in if you're happy :)