Support loading external Lua scripts

[redstrate]

This would allow maintaining a separate set of Lua scripts outside of Kawari. There has to be some thought put into this because:

• Currently we depend on Init.lua and Global.lua to do some leg-work, should those be replicated in mods?
• Supporting additional events is easy since those are loaded based on filename, but how does that work for actions/effects/etc?
• Documentation needs to be written so people know how to take advantage of this