Added two new import Fields

I modified the plugin to add two other woocommerce fields, total_sales and  post_date, to the import. I also made 'Custom Field' the default instead of 'Do Not Import' and set 'First Row is Header Row' option checked by default.

total_sales: This is the field that "sort by popularity" uses, setting it allows the "popularity" to be controlled.

post_date: Just like total_sales above, but controls the order of "sort by newness". The date needs to be in the format "2000-01-01 00:00:00" or it will give an error.