Add a config option for dropping obsolete quests

[noisiver]

This needs testing and I don't have the time to do it right now. Until that has been done it shouldn't be merged.

I decided not to add a print when the option is disabled since it runs every so often and it would most likely get very annoying.

This is helpful when completing quests for reputation and so on. The quests are often grey/gray when doing it and they'll constantly drop them unless it's possible to disable it.