[PROPOSAL] `availablePixelRatios`

# tl;dr

Everybody does not generate images for every `{pixel_ratio}` on Earth.

# Proposal

```js
new Imager({
  availablePixelRatios: [1.3, 2]
})
```

`{pixel_ratio}` will be replaced as:
- `` if `window.devicePixelRatio` equals `1` or is nonexistant 
- `-1.3x` if `window.devicePixelRatio` equals `1.3`
- `-1.3x` if `window.devicePixelRatio` equals `1.1`
- `-2x` if `window.devicePixelRatio` equals `1.5`
- `-2x` if `window.devicePixelRatio` equals `2`

Like `availableWidths`, it should match the equal or closest upper ratio available.