Render a referral URL of current page or from the given URL.

Attributes:

• url – URL to append the referral code. If empty, will render the URL of the current page.
• current_user – Append referral code of the current logged in user. Accepts: yes, no Default: yes
• user_id – Append referral code of a specific user.

Example:
[gamipress_referral_url url="http://website.com/"]

Example of use inline:
Your referral URL: [gamipress_referral_url]

Example of use on a link:
<a href="[gamipress_referral_url url="http://website.com/"]">Refferral link</a>