`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.
Merge pull request #35 from prism/chunk-load

`getChunk` clarifications, `loadChunk` method