Refactor XHR interceptor

In preparation for future work on the XHR interceptor.

Bug: twpowertools:153
Change-Id: Id8df1486c033ba02429a17d161e2bcc87a0f1de5
diff --git a/src/common/contentScriptsUtils.js b/src/common/contentScriptsUtils.js
index cb7a67d..51257c0 100644
--- a/src/common/contentScriptsUtils.js
+++ b/src/common/contentScriptsUtils.js
@@ -10,8 +10,12 @@
   injectStylesheet('data:text/css;charset=UTF-8,' + encodeURIComponent(css));
 }
 
-export function injectScript(scriptName) {
+export function injectScript(scriptName, prepend = false) {
   var script = document.createElement('script');
   script.src = scriptName;
-  (document.head || document.documentElement).append(script);
+  const root = (document.head || document.documentElement);
+  if (prepend)
+    root.prepend(script);
+  else
+    root.append(script);
 }