Implement loading of level files

[Jigoku]

Currently, enemies spawn on a timer, choosing a random group.

In future, groups of enemies should be triggered by the percentage/progress (eg; 0.00%).
Find a way to make levels that can be loaded easily.

Psuedo code example that may work:

level1.lua

level = { 
[1] = { "1.50", "delta" } -- spawn delta group at 1.50%
[2] = { "2.00", "dart" } -- spawn dart group at 2.00%
[3] = { "5.20", "tri" } -- spawn tri group at 5.20%
["boss1"] = "abomination" -- trigger boss event at end of wave 1
["boss2"] = "large" -- trigger boss event at end of wave 1
["next"] = { level2.lua } -- load the next level file when 100% is reached
}