Add support for ARFF and CSV files

You can now use ARFF and CSV files as feature files, and `skll_convert` can convert between those formats.

The only thing that might annoy anyone is that `tsv_label` has been renamed `label_col` across the board because of this, although we support the old name in config files. It's only people messing with the API directly who will be annoyed (i.e., @mheilman)