feat: add hook for customizing injected runtime tags

Enable other plugins to tap into the tag injection mechanism in order to
customize the functionality of lazy loaded chunks.

Closes #40.

Author: Patrik Sletmo

Diff for: package-lock.json

@@ -42,6 +42,7 @@
     "loader-utils": "^1.1.0",
     "normalize-url": "1.9.1",
     "schema-utils": "^1.0.0",
+    "tapable": "^1.1.3",
     "webpack-sources": "^1.1.0"
   },
   "devDependencies": {

Diff for: package.json

@@ -4,6 +4,7 @@ import webpack from 'webpack';
 import sources from 'webpack-sources';
 
 import validateOptions from 'schema-utils';
+import { SyncWaterfallHook } from 'tapable';
 
 import CssDependency from './CssDependency';
 import schema from './plugin-options.json';
@@ -24,6 +25,8 @@ const REGEXP_NAME = /\[name\]/i;
 const REGEXP_PLACEHOLDERS = /\[(name|id|chunkhash)\]/g;
 const DEFAULT_FILENAME = '[name].css';
 
+const compilerHookMap = new WeakMap();
+
 class CssDependencyTemplate {
   apply() {}
 }
@@ -127,6 +130,17 @@ class MiniCssExtractPlugin {
     }
   }
 
+  static getCompilerHooks(compiler) {
+    let hooks = compilerHookMap.get(compiler);
+    if (!hooks) {
+      hooks = {
+        customize: new SyncWaterfallHook(['source']),
+      };
+      compilerHookMap.set(compiler, hooks);
+    }
+    return hooks;
+  }
+
   apply(compiler) {
     compiler.hooks.thisCompilation.tap(pluginName, (compilation) => {
       compilation.dependencyFactories.set(
@@ -376,6 +390,9 @@ class MiniCssExtractPlugin {
                         '}',
                       ])
                     : '',
+                  MiniCssExtractPlugin.getCompilerHooks(
+                    compiler
+                  ).customize.call(''),
                   'var head = document.getElementsByTagName("head")[0];',
                   'head.appendChild(linkTag);',
                 ]),