Woocommerce WooSlider 2.5.0

* Update - Remove WooThemes Update Plugin reference.
* Tweak - WP 5.6 compatibility.
* Tweak - WC 5.0 compatibility.

* Fix - Fixes lazy loading output by adding optional attr filters and jquery filter for start callback.
classes/class-wooslider-frontend.php,
classes/class-wooslider-sliders.php

* Fix - Updated to use the latest version of FlexSlider, 2.6.3, fixes fade animation bug.
assets/js/jquery.flexslider.js
* Fix - Fixes handling of comma separated tag string when the output function is used in a theme (eg. Canvas).
classes/class-wooslider-sliders.php