#150 Add configurable auto-save functionality

[bovineaux]

Auto-save functionality configurable via exult.cfg which allows saved games to be made automatically at specified intervals.

Perhaps with the following config options, in what I think is most to least important:

- enable / disable
- number of minutes (seconds?) between saves
- amount of auto-saves to keep (or 0 to not delete old ones?)
- pattern / expression to use for autosave name (e.g. <pattern>Auto: %avatar_name% @ %play_time%</pattern> = "Auto: Avatar @ 1h 23m")

Could be very useful for debugging and crash recovery

Sorry if this exists already, I checked the docos and didn't see it