Improve handling of referrer

[bpugh]

It looks like right now aside from the configuration options, the referrer is only being set from document.referrer which works when the user gets to the site from an external source but not when they are navigating around the site. Also document.referrer doesn't change when navigating within Gatsby so if a user visits multiple pages they will all show the same external referrer. I don't think this is the behavior we want and it's not how it would behave on a traditional non SPA website.

Fortunately, I think an easy fix is we use the prevLocation parameter of onRouteUpdate. If it's not null then we send that as the referrer otherwise we send document.referrrer. (https://www.gatsbyjs.com/docs/browser-apis/#onRouteUpdate)

Something like:

settings.referrer = () =>
      window.goatcounter?.get_query?.('ref') ??
      window.goatcounter?.get_query?.('utm_source') ??
      prevLocation?.pathname ??
      document.referrer;

I'd be happy to submit a PR but wanted to get your thoughts on it first.

Thanks for this handy plugin!