Grayscale images aren't saved as 8-bit PNGs or JPEGs

# Expected Behavior

When writing images to disk after calling `greyscale()` method should produce 8-bit PNGs and JPEGs.

# Current Behavior

Even when `greyscale()` is called images are still 24 or 32 BPP. After some digging I've discovered that you can get 8-bit PNGs by leveraging the `colorType()` method. Unfortunately no such thing for JPEGs.

## Steps to Reproduce

```javascript
Jimp.read('image.png').then(image => {
image.greyscale().write('image_greyscale.png');
});

Jimp.read('image.jpg').then(image => {
image.greyscale().write('image_greyscale.jpg');
});
```

## Context

- Jimp Version: 0.14
- Operating System: Windows 10
- Node version: 12