Add basic configuration support

[tung]

There needs to be a way of saving data outside of the save file system, for e.g. controls and screen configuration. The implementation of this should be roughly the same as for save files, except saving and loading occurs in the one configuration file.

[tung]

Add config save/load API to game.lua. Closed by b731a3d.

• Try ./game test/config.lua a few times to test it.
• New game.lua functions: game.loadConfig(), game.saveConfig().