New Dev Menu Option: "(Shift+D) Run Debug Script"

This new option uses loadfile() to execute: ../lua/debug_script.lua

So instead of using the debug console developers can now edit and
execute this file in their programming IDE while CorsixTH is running,
using the underscore global variable to refer to clicked entities
Merge pull request #290 from J-Shep/run_debug_script

New Dev Menu Option: "(Shift+D) Run Debug Script"