Adding the embeddable promote MDN JavaScript code.

Submitting the promote MDN script again because I messed up with the branch with the last PR (#2111).

The code to be embedded into websites to include the Promote MDN script is:

    <script>
        (function (d,f,a) {
            // Your settings here
            var o = {maxlinks: 4, includeElems: ['div', 'h'], linkClass: 'link-to-mdn', extraLinks: {'keywordx': 'http://example.com'}};
    
            var s=d.createElement("script");s.type="text/javascript";if(s.f)
            {s.a=function(){if(s.f=="loaded"||s.f=="complete")
            {s.a=null;mdnPromoteLinks(o)}}}else{s.onload=function()
            {mdnPromoteLinks(o)}}s.src="promote-mdn.js";
            d.getElementsByTagName("head")[0].appendChild(s)
        })(document,'readyState','onreadystatechange');
    </script>

Lots of thanks to @darkwing for his patience with my last PR and helping me make a lot of improvements. You are my guru :bow: And thanks to @groovecoder for his original Promote MDN Wordpress plugin which this script is based upon.