Difficulty based mining speed

Allows a more challenging experience for players on hardcore/hard difficulty, and make it easier for players on easy/peaceful difficulty.

Summary:

• Hard difficulty: 0.7x default mining speed;
• Normal difficulty: default mining speed;
• Easy & Peaceful: 1.3x default mining speed;

Allow the player to opt out by making this a game rule: diff_based_mine_speed: true or false.