Added an option to choose the csv delimiter on export.

Hi!
I'm leaning the system, and have encountered the need to change the CSV delimiter, so Excel (yeah only excel is weird enough to not try to detect the delimiter) can open the file correctly. 

I just made this small pull request, I would like feedback if there are better ways to do it, as I'm still trying to learn Node and Keystone. Maybe expose the whole set of options offered by node-csv? Allow per model config instead of a global config? ( but I don't think that it really make sense). I Didn't know where I should document it, so didn't add it to the doc.

Thanks!