[PROPOSAL] `data-src` + pixel ratio variable

# The intention

Imager takes care only about widths, not about pixel ratio. 


# The proposal

Exposing a `{pixel_ratio}` variable, as it is proposed with `{width}` (cf. #21).

From:
```html
<div class="delayed-image-load" data-src="Assets/Images/Generated-{pixel_ratio}x/A-{width}.jpg" data-width="1024"></div>
```

To:
```html
<div class="delayed-image-load" data-src="Assets/Images/Generated-1x/A-1024.jpg" data-width="1024"></div>
```

Caveats to solve:
- [x] some URI structures have pixel ratio prefix, only when the ratio is greater than 1 (like `Assets/Images/Generated/A-1024.jpg` and `Assets/Images/Generated-2x/A-1024.jpg`) — do we want to handle that?
- [x] some pixel ratios are float values (like 1.3 or 1.5), should we keep them as is or floor/round those numbers?