Added optional TUNDRA_LUA_PATH so that tundra can include additional script files from a user defined directory

Tested on Windows + Linux
The path should separate paths with semicolon as described in http://www.lua.org/pil/8.1.html

Usage:
Create a file ~/tundrascripts/foo.lua
Set the environment variable: export TUNDRA_LUA_PATH=~/tundrascripts/?.lua
Now foo.lua can be included in any tundra script with a: local foo = require('foo')