`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.
From the docs, it sounds like `getChunk(x, z)` and `getChunk(x, z, false)` behave differently. Is that correct?

How I interpreted the docs:

* `getChunk(x, z)` returns only currently loaded chunks,
* `getChunk(x, z, false)` returns loaded or unloaded if already generated
* `getChunk(x, z, true)` returns loaded or unloaded, even if it's not already generated