Adds extra argument for appending classes to the img tag returned

[brob]

While implementing this on my site, I realized there wasn't a way to add a class to the tag rendered by the plugin (which I use classes on img a decent bit).

I'm not a huge fan of adding a ton of arguments to universal shortcodes, but I feel like an additional optional classname at the end wouldn't be too far and would extend the functionality a lot.