fixed difficulty distribution

[RasmusAntons]

This adds the option to configure a fixed distribution of goals for a difficulty. This is a suggestion for #14

For example, hard.json can look like this:

{
  "distribution": [
    0.0,
    0.2,
    0.4,
    0.4,
    0.0
  ],
  "number": 3
}

A game using that difficulty will consist of 20% easy, 40% medium and 40% hard goals.
This file can also be generated with absolute 5x5 numbers:

BingoDifficulty.builder(BingoDifficulties.HARD).number(3).distribution(0, 5, 10, 10, 0).build()

Caveats:

• Perhaps it would be more useful to have a startup parameter instead so users don't need to create a datapack to use this.
• It might be good to have some random variation parameter.
• I don't know the code well and just hacked this in.