Add compatibility mode options to config

- Add a new example config file compat.cfg to attempt compatibility with
Minecraft 1.7 servers
- Add new [global] options section, for options applying to both
treasure hunts and to dungeons.
- Add 'compat' option (usually false)
- Add tweaks in Landmarks code to disable the few places which use
1.8-specific features is the compat option is True.
- Fix for chest hiding in empty graveyards