feat(ordering): add support for insertInto option

This adds an new `insertInto` option that is optional and backards-compatible.  Specifying a CSS selector allows the user to specify a DOM node into which the async-loaded <link> tags should be inserted.

Closes webpack-contrib#370, related to webpack-contrib#328,webpack-contrib#331

Author: brophdawg11

src/index.js

@@ -132,6 +132,9 @@ class MiniCssExtractPlugin {
         );
       }
     }
+    if (!this.options.insertInto) {
+      this.options.insertInto = 'head';
+    }
   }
 
   apply(compiler) {
@@ -378,8 +381,11 @@ class MiniCssExtractPlugin {
                         '}',
                       ])
                     : '',
-                  'var head = document.getElementsByTagName("head")[0];',
-                  'head.appendChild(linkTag);',
+                  `var parent = document.querySelector("${
+                    this.options.insertInto
+                  }");`,
+                  'if (parent) { parent.appendChild(linkTag); }',
+                  'else { reject("Invalid parent element"); }',
                 ]),
                 '}).then(function() {',
                 Template.indent(['installedCssChunks[chunkId] = 0;']),