Invalid sourcemaps for sass when adding IconfontWebpackPlugin
## Expected Behavior
Plugin does not affect sass sourcemaps

## Actual Behavior
Breaks sass sourcemaps

## Steps to Reproduce the Problem
  **Sass sourcemap without IconfontWebpackPlugin**
  1. clone [config-example](https://github.com/AliekseiDev/bug-iconfont-webpack-plugin-example)
  2. npm i
  3. npm start
  4. open index.html from dist folder and inspect div.hello and span.icon

  **Sass sourcemap with IconfontWebpackPlugin**
  1. uncomment line 35 in webpack.config.js
  2. uncomment lines 3-7 in hello-icon.scss
  3. npm start
  4. open index.html from dist folder and inspect div.hello and span.icon

  The sources with IconfontWebpackPlugin are messed

  
## Example Link
* Link: [config-example](https://github.com/AliekseiDev/bug-iconfont-webpack-plugin-example)