Add a default mapping for the .tsv extension to point to okf_table_tsv

[Chase Tingley]

Even though TSV is a variant of CSV, some people use the .tsv suffix specifically for TSVs, to distinguish them from CSVs. (For example, .tsv is listed as an affected for the file type [in Wikipedia(https://en.wikipedia.org/wiki/Tab-separated_values)).

Currently, we don't register a mapping for .tsv to the okf_table_tsv filter in Rainbow etc. We should, since it wouldn't conflict with anything.

[YvesS]

Added extension mapping for .tsv in rainbow. This resolves issue #683. Also added mappings for .csv and .markdown.

→ <<cset dff16402a266>>

[YvesS]

I've added the mapping (and one for .csv too). But I think I recall why we didn't have one: It's because most of the time only specific fields of the CSV and TSV files is extractable, so the default configurations are not very useful. But I suppose it doesn't hurt to have the mapping.

[Chase Tingley]

Thanks Yves!