[Dice] Extra modifiers

[rakiru]

• Exploding dice modifier ! - If a roll is the max, roll again and add to original dice, repeatedly.
• Numeric operator modifiers +-*/ - Do maths things to total.
• Comparison operator modifiers > >= < <= == - Filter rolls by numeric comparison. == or = for equals?

Syntax/parsing may need a complete overhaul (full on parse tree?). It's currently just a simple implementation of what python dice libraries I found provide, but written myself to add configurable limitations. Actually...

• Add modifier count limit config option