Add turf.difference and turf.union

[ammarkhan97]

Hello, this port is very nice overall but it would be nice if it could have the difference and union transform operations. We have a product using turfJs and we have a new product that is processing a shape file and converting it to t geoJson. Part of this is calculating the area, we want to match the area calculation algorithm to match our js implementation but the lack of the difference and union operations is preventing us from doing so at the moment.