Running through rollup for node causes issues with dynamic require

Thanks for a neat little library!

I was attempting at incorporating it into a library of mine which I bundle using rollup and I get an issue
````
Error: Dynamic requires are not currently supported by rollup-plugin-commonjs
````
which was related to [this line of your library](
https://github.com/developit/web-worker/blob/master/node.js#L190)

From what I can tell it looks like it's to do with requiring the URL of the worker?

Anyway just thought I'd flag it as other users of rollup may run into the same issue.

Cheers
Rowan