Update helperfunctions.php

[summersab]

• Added foxyshop_product_custom_fields to return custom fields from the product listing
• Added foxyshop_product_variation_count to return the number of variations for a product
• Added foxyshop_product_variation_names to get an array of the variation names
• Modified foxyshop_product_variations to:
  1. permit retrieving a single variation field and
  2. make the variation name a placeholder value in the form element vs a label (applicable for select, text, and textarea elements)