Shortcode for total # of visitors (e.g. a site counter)

This PR provides the ability to add the total # of visitors as a shortcode. You can use it in your site (say in footer.php) with the following:

<span>
  Visitors:
  <?php
    echo do_shortcode('[koko_analytics_site_counter days=30]');
  ?>
</span>

If there's support for the PR I'm happy to clean it up, provide unit tests, support for pageviews vs visitors, etc.

Thanks!

Hey @dannyvankooten, this PR has been sitting for awhile. Do you mind taking a look and giving feedback?