Support easy integration of Google Analytics

[damithc]

Provide an easy way to add Google Analytics to a site, e.g., simply by configuring site.json

[crphang]

@damithc Read through the codebase on supporting this, it is actually already supported if we add google-analytics tracking code into externalScripts of site.json.

All externalScripts from site.json are added when calling generatePages -> createPage using ejs template.

Should we have a separate key for Google Analytics to make it more explicit, or continue using externalScripts, but also mention this in the User Guide?

[damithc]

Yes, it can be done already, but we can make it easier. For example, we can allow users to add GoogleAnalytics tracking to a site by simply adding the tracking ID to site.json.
Example from a similar product https://gohugo.io/templates/internal/