added .csv support

To supplement the other spreadsheet support we have (`.xls` and `.xlsx`), I thought I'd throw this together really quickly. I was hoping to be able to [automatically dectect which delimiters are used](http://www.dotnetperls.com/csv) but that was failing miserably on the `tests/csv/raw_text.csv` test. If anyone has any insights into how to get that to work, I'd greatly appreciate it!