Update bike-index-listings.php

Fixed prior bug in lines 159-161 that prevented ZIPCODE from ever saving in admin side widget settings. 
This meant that bikes weren't getting geocoded and were pulling totally randomly when deployed to client websites.
Added support for "show stolen only" admin settings in checkbox - i.e. checkbox to click for showing only STOLEN bikes in the widget.
This adjusts the API call to bikeindex using stolen => true