Insert scripts after meta tags

At the moment the build system inserts OpenForge scripts tags directly after `<head>` in all html files the extension uses. This prevents extensions from using the latest IE version in the popup window by inserting a `<meta http-equiv="X-UA-Compatible" content="IE=edge" />` tag. IE only recognises this tag properly if it comes before anything else in the head.

This patch makes sure the OpenForge scripts are only inserted after any meta tags in the page head.

See also #30 for a related issue involving the background page.