Track subsequent visits automatically

[bilbof]

Perhaps we should track subsequent visits and any utm codes by adding this to the LocalStorage object:

    "visits_before_conversion": 2,
    "visits": [
      {
        "referrer": "news.ycombinator.com",
        "utm_source": "hn_post"
      }
    ]

Each time a visitor comes to the site, we iterate visits_before_conversion and add a visit to visits.

This is based on feedback from a Hacker News comment:

This is pretty cool, but it seems like it doesn't store data for multiple visits. Say I land via Google yesterday, come back today via HN, then sign up. I'd like to know all that.

[bilbof]

Added in Version 1.1