Add ability to include Google analytics scripts in IG pages

[ElliotSilver]

To track page views, add following script just after </head> on every page of the IG.

<!-- Google tag (gtag.js) -->
<script async src="https://www.googletagmanager.com/gtag/js?id=G-ABCDE12345"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'G-ABCDE12345');
</script>

G-ABCDE12345 should be replaced by a variable that can be specified in features.yml (?)

[costateixeira]

if we do this I would just make sure it is conditional to the existence of the google analytics ID, to make sure templates or IGs that don't provide the ID would not have broken javascript

[ElliotSilver]

@costateixeira, of course. That would also give people who don't want google tracking to avoid it.

[ElliotSilver]

This is a followup to HL7/fhir-ig-publisher#383 which was interpreted to only request a way to specify an ID, not to actually specify and use it.

[grahamegrieve]

This is a template thing, nothing to do with the publisher

[costateixeira]

Transferring to https://github.com/HL7/ig-template-base, where this belongs