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

src/index.js

@@ -113,6 +113,7 @@ class MiniCssExtractPlugin {
     this.options = Object.assign(
       {
         filename: '[name].css',
+        insertInto: null,
       },
       options
     );
@@ -378,8 +379,10 @@ class MiniCssExtractPlugin {
                         '}',
                       ])
                     : '',
-                  'var head = document.getElementsByTagName("head")[0];',
-                  'head.appendChild(linkTag);',
+                  `var selector = "${this.options.insertInto}";`,
+                  'var parent = selector && document.querySelector && document.querySelector(selector);',
+                  'if (parent) { parent.appendChild(linkTag); }',
+                  'else { document.getElementsByTagName("head")[0].appendChild(linkTag); }',
                 ]),
                 '}).then(function() {',
                 Template.indent(['installedCssChunks[chunkId] = 0;']),