Add option for custom separator when exporting to csv

[ilv]

It would be very useful tu have an option for choosing a different separator symbol rather than a comma (e.g. ';'). When survey's questions and answers have commas in it, the csv turns out very messy and hard to manipulate with office-like software.

thanks!

[simonv3]

Thanks for opening the issue @ilv!

This is kind of related to #54 in which I suggest that CSV content should probably be escaped.