{width} Interpolation in data-src

# tl;dr

`data-src` does not always comply with `{width}` being replaced by a numeric value.  
`{width}` is now replaceable any width breakpoint by any value of your choice using the `availableWidths` option.

Relies on #15 to be merged.

# Features

## data-src
* `{width}` can be replaced by something else than a numeric value

`<div data-src="http://farm4.staticflickr.com/3773/9676470682_3d418eeb40_{width}.jpg">` -> :
 * `http://farm4.staticflickr.com/3773/9676470682_3d418eeb40_d.jpg`
 * `http://farm4.staticflickr.com/3773/9676470682_3d418eeb40_z_d.jpg`
 * `http://farm4.staticflickr.com/3773/9676470682_3d418eeb40_b_d.jpg`

## Configuration

The array of widths now accepts array-like values:
* without interpolation: `availableWidths: [100, 250, 500]` will replace `{width}` by either `100`, `250` or `500` 
* with interpolation: `availableWidths: [ [100, 'small'], [250, 'medium'], [500, 'large'] ]` will replace `{width}` by either `small`, `medium` or `large` 


# Demo

A demo showcases this using Flickr URL system.

![](http://i.imgur.com/u4oBa.gif)