Add difficulty level.

The difficulty setting is an integer: 0 is easy, 1 is medium, 2 is hard.

Alternatives considered:

* Having 1 be easy, 2 be medium, 3 be hard - I found that I used arrays in games to define settings based on difficulty level, which led to me constantly indexing the arrays using `difficulty - 1`, which was annoying.
* Using strings instead of integers, e.g. `"easy"`, `"medium"`, `"hard"` - It might be easy to typo this? And it's not internationalizable? Honestly, I have no idea. It might actually be a better approach; that way games would be encouraged to use objects to define difficulty settings instead of arrays, which is more obvious and readable.