Add TypeScript declaration file (faceyspacey#90)

Author: samijaber

index.d.ts

@@ -0,0 +1,25 @@
+declare module 'extract-css-chunks-webpack-plugin' {
+  import { Loader, Plugin } from 'webpack';
+
+  class ExtractCssChunksPlugin extends Plugin {
+      static loader: Loader;
+
+      constructor(options?: ExtractCssChunksPlugin.PluginOptions);
+  }
+
+  namespace ExtractCssChunksPlugin {
+      interface PluginOptions {
+          /**
+           * Options similar to those of `mini-css-extract-plugin`
+           */
+          filename?: string;
+          chunkFilename?: string;
+          /**
+           * we try to automatically inject hot reloading, but if it's not working, use this config
+           */
+          hot?: boolean;
+      }
+  }
+
+  export = ExtractCssChunksPlugin;
+}