Have a way to refer to the default target? #141
Description
Following https://kg-construct.github.io/rml-io/spec/docs/#logical-target-in-rml and https://kg-construct.github.io/rml-io/spec/docs/#multiple-targets, it's not possible to specify that I want triples to be put in both the default target and a custom target, as there's no construct to specify the default target
Should we introduce something like rml:defaultTarget?
cfr. #131