Support external and absolute URLs in import and url plugins

For example, it's now possible to have @import url('http://fonts.googleapis.com/css?family=Lato:300,400,700');

In addition, convert url(image.png) => require("./image.png") the way css-loader does.

The methods for testing and converting the URLs can be found here
https://github.com/webpack/loader-utils/blob/db3c7cd123355f61846497ebeef614670c075ed9/index.js#L93-L142

See the css-loader for reference
See https://github.com/webpack/css-loader/blob/70771dc2586d90439b370c6d3b5921e48058a4fe/index.js


Not sure if this is backwards compatible with the current version of rework-webpack-loader or not.
this looks the same as the one in [`lib/plugins/urls.js`](https://github.com/aaronj1335/rework-webpack-loader/blob/54c5c39d3c1e92882d1d280f84c8eaaca0e92128/lib/plugins/urls.js#L13), so it should be in a shared module. i'b be cool with like a `./../patterns` module or something.