tsv-summarize: "total line" when grouping.

[Llammissar]

Only a small ER this time. ;)

I hit a case where I'd like to have a "total" line in addition to the output of my --group-by. I can get it by removing the --group-by option and rerunning the input, but it'd be nice if I could get them both in one go.

[jondegenhardt]

Makes sense. Not sure if it's a small ER or not :) Depends on how I structured that part of the code. Hopefully small. I'll look into it.