Multi-valued subject maps

[dachafra]

issue: no support for multi-valued Subject Maps

suggestion: add support for multi-valued subject maps. This can be useful in some cases, for example when it is not possible to specify an iteration that separates subject values, like with pipe separated values in a CSV source.

[dachafra]

I think the solution be provided for collection/containers is going to work as well with this problem, right @chrdebru? Can we close this?