add HTML syntax for Snipcart

_includes/product-definition.html

@@ -2,6 +2,7 @@
  class="button primary fit snipcart-add-item"
  data-item-id="{{ page.product_id }}"
  data-item-name="{{ page.heading }}"
+ data-item-url="{{ page.url }}"
  data-item-price="{{ page.unit_price | precision: 2}}"
  data-item-image="{{ page.banner_image }}"
  data-item-description="{{ page.content | remove: '<p>' | remove: '</p>' }}"