Add fast flexible formatting to `lfreq`

In spite of the easy tests/lfreq.awk awk 1..2-liner, for which lfreq is mostly just an optimization / demo of adix/oats, maybe given how many people seem surprised at uniq -c and sort-based histogramming, Unix was always missing this slightly more specific utility. So, make this particular realization a little nicer with fairly general user formats of output.

This could be used, for example, to do git log --author=WHOEVER --date=raw | awk '{print int($2/86400/91)}' | lfreq -n0 -f@k,@c { maybe also piped to a chart/plot program after lfreq } to assess quarterly git activity of some author.