`getChunk` clarifications, `loadChunk` method

Based on @mikeprimm feedback on c34d11d0, renaming chunk coordinates in method sig to `cx`, etc.

Clarifying that `getChunk` returns null if it's not loaded.

Adding a `loadChunk` method, with optional bool to disable generating.
The issue I have with API like this comes down to it inherently masking what it does.

While the docs explain it quite well, I tend to think it is better to force them to specify the boolean. That way the plugin developer is forced to know exactly what is going to happen.