Add support for the flatpickr plugin

I was able to integrate the flatpickr plugin and It looks nice, but I won't drop support for the Timepicker Addon until I'm sure that other plugins are working fine, for now, I'll leave it to "Needs Review" just in case someone wants to have a peek on this.

The other plugins seem to work fine:

I updated the README.md with extra instructions for installing the new library, I think I'll merge this to the 3.0.x branch and then create another MR for the 8.x-2.x branch.

Merged to the 8.x-2.x branch, works as expected as well:

I think I'll merge this and move it to "Fixed", I'll delete the Timepicker addon plugin later.

Automatically closed - issue fixed for 2 weeks with no activity.