[Google Analytics +] How to enable "Opt-out of tracking for your site" for a visitor?

I am preparing for the European privacy legislation commencing 25 May, and the consequences for Google Analytics.
How can I allow visitors to choose to opt out?

Are you able to implement this feature in Google Analytics +?
Right now the only option is to add this snippet
window['ga-disable-UA-XXXXX-Y'] = true; on site.

No option to allow users to opt-in/out this.

Thank you