Added option to preselect variable option

I've added this functionality for people to create URLs which automatically select the variable pricing option.

It looks for `price_option` added to the URL using `$_GET` and then matches the value from the GET variable to the variable price option and automatically selects that radio button.

See here: https://easydigitaldownloads.com/support/topic/link-to-product-with-price-option-preselected/.