[PROPOSAL] `data-src` + value interpolation

# The intention

Imager replaces the `{width}` variable by a numeric value/regular expression. *User context might not use numeric values in responsive images filepaths* (like Flickr, the Guardian or any user using `grunt-responsive-image` with a string suffix).


# The proposal

The user can configure an alternate replacement for a specific width:

From:
```javascript
var imgr = new Imager({
  availableWidths: [[320, 'small'], [640, 'medium'], [1024, 'large']]
});
```

To:
```html
<div class="delayed-image-load" data-src="Assets/Images/Generated/A-large.jpg" data-width="1024"></div>
<div class="delayed-image-load" data-src="Assets/Images/Generated/B-large.jpg" data-width="1024"></div>
<div class="delayed-image-load" data-src="Assets/Images/Generated/C-large.jpg" data-width="1024"></div>
```

Eventually people can mix them the way they want:

```javascript
var imgr = new Imager({
  availableWidths: [[320, 'small'], 640, [1024, 'large']]
});
```