Simple TSV output format

I've added a `@tsv` output filter by copying the `@csv` filter. It outputs into TSV (tab-separated values) file. Some people have asked for this on #48 .

I haven't done anything with string escaping, because (a) I'm not sure how to use `escape_string`, and (b) I'm unsure what "the standard" TSV approach is. I think many unix tools don't escape tabs.