Create LevelScalingModifierPlugin.rb

First revision of a plugin for expanding level-scaling modifiers, e.g.:
```
"ApplyModifier"
{
  "Target"		"CASTER"
  "ModifierName"	"mod_lvl1 mod_lvl2"
}
```
becomes
```
"Random"
{
  "Chance"    "0 100"
  "OnSuccess"
  {
    "ApplyModifier"
    {
      "ModifierName"    "mod_lvl2"
      "Target"    "CASTER"
    }
  }
  "OnFailure"
  {
    "ApplyModifier"
    {
      "ModifierName"    "mod_lvl1"
      "Target"    "CASTER"
    }
  }
}
```