Added support for Gtag and Google Tag Manager

I received some feedback on Twitter last week from users who installed Google Analytics through Gtag.js and Google Tag Manager.

The initial version of IpMeta.io only supported the older Analytics.js implementation, so they were unable to install IpMeta on their website.

I have made some changes to the Plugin to also be able to support the Gtag.js and Google Tag Manager implementations.

Installation instructions have been updated accordingly.