rss2i.py: Add switch for enabling .csv output

The normal behaviour of the download and the restore command is to output tab-separated, so called .tsv, data files. Since Matlab does not interpret this format automatically, it would be nice to have a switch that enables the generation of .csv, i.e. comma-separated, output files for both commands.

Desired command line

It would be sufficient to have a --csv switch without any short option. Then

rss2i -a download -d 1,2 -f config.cfg -o test --csv

should output two .csv files.