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

[intentionally-left-nil]

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!

[intentionally-left-nil]

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

[dannyvankooten]

Hey @intentionally-left-nil,

I'm so sorry for not seeing this before and not getting back until now. I really like the functionality you added here and hope to be able to take a closer look and merge this real soon. The final naming of things might differ a bit, but rest assured that the functionality will end up in the core plugin!

[dannyvankooten]

It's finally merged @intentionally-left-nil! Thank you for being so patient.

[intentionally-left-nil]

Sweet!