YITH WooCommerce Membership Premium 2.17.0

* New - search for membership by user fields
* Update - plugin framework
* Dev - added membership object in emails

* New - support to WooCommerce 3.6
* New - support to YITH Mailchimp 2.1.1: unsubscribe email when the membership expires or is cancelled
* Tweak - fixed notices for membership emails
* Update - language files
* Update - plugin framework

New - search memberships by order fields
New - set a list of excluded membership plans in discount rules of YITH WooCommerce Dynamic Pricing and Discount
Fix - protected media download issue
Fix - js error in Membership plans
Tweak - prevent issues in combination with some plugins using the_title filter such as WooCommerce PayPal Checkout Gateway
Update - language files
Update - plugin framework