feat: output `srcset` as a custom attribute

[heyitsbryanm]

Is your feature request related to a problem? Please describe.

Some CMS components require srcsets to be output as a custom attribute for other files or scripts.

For example, the Cornerstone theme for the BigCommerce CMS uses a data-image-gallery-new-image-srcset attribute to swap out the srcset of the main image in a product display page:

Describe the solution you'd like

A way to pass an imgix.js generated srcset to a custom HTML attribute.

An example solution:

1. Create custom configuration - ex: imgix.config.customSrcsetAttribute = 'data-image-gallery-new-image-srcset';
2. A srcset could then be generated with <img data-image-gallery-new-image-srcset="http://bryansandbox.imgix.net/_static/images/img02.jpg"/>