v2.4

Fix Uncaught Exception: Serialization of 'Closure' is not allowed when creating button with closure callback.

v2.3

• Custom button adding function introduced.
• PHP min version changed to 7.1.

v2.2.1

== Changelog ==
= 2.2.1 =

• Nonce verification for ajax queries.
• PayPal link updated.
• Plugin tags updated.