Set free shipping threshold based on country/market#397

Be able to set the minimum free shipping threshold to COUNTRY/MARKET, rather than just one min spend. For e.g. spend $150 AUD or $150 USD to get free shipping (not $150 AUD which is store currency, converted to USD, which is $99 USD and significantly less than the free shipping rate) / not sustainable when shipping to multiple countries from one location).

e.g. AUD: 150, NZD: 150, USD: 150, CAD: 150, SGD: 150, EUR: 150.