Add track_pageview option to make it easier to use with Turbolinks

This pull request adds a `track_pageview` option (which is `true` by default, to keep existing behavior). When used, it does not outputs the `ga('send', 'pageview')` line in the tracker code.

We’re using this to make the middleware play nice with [Turbolinks](https://github.com/rails/turbolinks) which does not evaluate `<script>` tags in the `<head>` when changing pages. This new `track_pageview` option allows us to have this code in the `<head>`.

```html
<head>
  …

  <script type="text/javascript">
    (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
    (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
    m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
    })(window,document,'script','//www.google-analytics.com/analytics.js','ga');

    ga('create', 'UA-0000000-01', {});
  </script>
</head>
<body>
  …
</body>
```

And manually track our pageviews in a `<script>` element at the end of our `<body>` (**which gets evaluated by Turbolinks when it changes the page content**)

```html
<head>
  …
</head>
<body>
  …
  <script>
    if (window.ga) { ga('send', 'pageview'); }
  </script>
</body>
```

What do you think? It doesn’t not anything if the option is not used :smile: