Allow comma separated CSV files

The separator selector in the CSV import dialog was unused. This passes
the value into the xslt and adds ',' as possible value.

I'm sure this could be done much better (pass the actual character instead
of the index), but I couldn't get that to work and this does seem to do
the trick.

Also added a test dive to test this feature.

Fixes #321

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>