Change CDN away from unpkg

[AndreasBakke]

Issue Summary:

unpkg is unmaintained, which may result in a bad user experience for users. Especially today, when unpkg returned 520 errors for most libraries. ref: mjackson/unpkg#338

Recommendation:

Change provider from unpkg to cdnjs, jsdelivr or equivalent.

Example:

<script src="https://cdnjs.cloudflare.com/ajax/libs/tagify/4.14.0/tagify.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/tagify/4.14.0/tagify.polyfills.min.js" </script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/tagify/4.14.0/tagify.css"/>

[yairEO]

Thanks I havn't noticed any problems whatsoever with Tagify on unpgk but if you say they are unmaintained then I will update the text in the README to recommend cdnjs.