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.
i was using `_` as just a placeholder, but since it's now got some meaning, maybe rename it to `declarationValue`?