clear up CSV/TSV confusion

[RichMorin]

The documentation and file extensions say various things about the format of the input files. It appears, however, that they are NOT in CSV format. How about changing everything to say "TSV" (or "tsv", as appropriate), to reduce the chance of confusion?

[jexp]

If you have a minute, I'm happy to accept a PR. It works with both formats (with a config option) though and currently doesn't look at the extension.