Add srcset functionality

If you use the `srcset` attribute in Chrome, the swap function won't work because Chrome disregards the `src` attribute in favor of `srcset`.  This PR adds the capability to optionally set the `srcset` attribute via string or array.

For the argument type (optional string or array of strings), I put `string|Array.<string>=` per guidelines provided by usejsdoc.org, but I noticed you've been using String (with a capital S) for your string primative arguments - happy to update the PR to conform to that if you'd like, though. 

Thanks for writing this plugin!