Add possibility to choose which fields are being exported in the subscribers export field.

There is still no possibility to define which of the fields are being exported into the *csv file when exporting subscribers of a certain event.

Producing lists on paper for events is still a big deal, even in these digital times. For some events people at the door still want to have a printed list.

There are way too many fields now when making a csv file, so it needs lots of time to edit such a list before it can be printed out and used.

If we could define which fields will be exported, the proces would be very smooth.