Make URL argument absolute (#154)

open-my-page-button/background.js

@@ -4,7 +4,7 @@ Open a new tab, and load "my-page.html" into it.
 function openMyPage() {
   console.log("injecting");
    browser.tabs.create({
-     "url": "my-page.html"
+     "url": "/my-page.html"
    });
 }