Wrap in UMD for CommonJS/AMD compatibility

[Lokaltog]

I'm using webpack for managing my assets, and while it's possible to shim this plugin so it works with CommonJS or AMD it would be great if it supported it natively. I realize this plugin aims to be very small, but the UMD overhead isn't very large and it makes it much more convenient importing it for any asset manager that uses CommonJS or AMD loading (it would also make the plugin compatible with Node). Here's a suggested UMD wrapper for dependency-free plugins that is compatible with both CommonJS and AMD plugin loading.