support rclone's bisync feature

[nioncode]

What's your idea?

Currently, ludusavi uses rclone sync to backup / restore savegames. This completely overwrites the state in the remote with the local changes.

It would be great if ludusavi would support rclone bisync instead, which would allows to change the savegame directory on multiple machines at the same time and then sync those changes on the next sync attempt without overwriting each other. This would prevent issues either:

• if one does not manage to backup their latest local savegames (because of some error), but then tries to restore the latest savegames in the future (I think ludusavi will error out in this case though?)
• if one plays on two devices at the same time (different games) and then sync both of them. I think this is not possible at the moment, because one of the backups will overwrite the other (or does ludusavi work around this?)

In any case, I think ludusavi works around some of the issues already, but maybe switching to bisync feature would simplify ludusavi's code, since it does not have to handle so many edge cases.