Updated `save-pixels` signature to match `get-pixels`. Fixes #6

As discussed in #6, it would be ideal to have consistent signatures between `get-pixels` and `save-pixels`. To keep consistency with `Canvas`, we are using `[width, height, channels]` for our `ndarray's`. In this PR:

- Upgrade to `get-pixels@3.0.1` to pickup latest signature
- Updated `README.md` with new `ndarray` signature
- Updated `save-pixels.js` to work with new shape