Add support for optgroup

The code that I've updated adds support for the optgroup tag when rendering the plugin from an inline select tag. I added two new classes (one for the optgroup row and one for sub-options of an optgroup tag) and some basic styling, but you might want to use a different style as a default in your plugin. data-imagesrc and data-description both work for all three types of elements.

I did not minify the code because I don't know what service/settings you use for your plugin so I figured I'd let you handle that.

There's also one typo/bug I fixed. It's on line 105 in my code (101 in yours), a missing > stops the code from working.