Support webpack 4 and Multiple compilations

💥 Breaking Change

• The plugin is now the main import of this package. In other words:
  • import SvgStorePlugin from 'external-svg-sprite-loader/lib/SvgStorePlugin'; is now import SvgStorePlugin from 'external-svg-sprite-loader';
  • const SvgStorePlugin = require('external-svg-sprite-loader/lib/SvgStorePlugin'); is now const SvgStorePlugin = require('external-svg-sprite-loader');
• To use the loader you should now always use SvgStorePlugin.loader. If you use the loader name like external-svg-sprite-loader then it won't work.
• The store object is no longer shared across multiple compilers and it is no longer a static property of SvgStorePlugin. It is now a property of an instance of SvgStorePlugin.

🚀 Feature

• Upgrade to Webpack 4
• Upgrade all dependencies
• Use new Tapable API
• Use iterables instead of forEach and map functions on Chunks and Modules
• Use mini-css-extract-plugin instead of extract-text-webpack-plugin
• Show warning when dimensions information is missing for an icon
• HMR
• In the example apps:
  • Rewrite React components as stateless components
  • Generate multiple sprites
  • Add server rendering