• csvtk grep: large performance improvement by discarding goroutine (multiple threads),
  and keeping output in order of input.
• Better column name checking and **stricter field checking,
  fields out of range are not ignored now.
  Affected commands include cut, filter, freq, grep, inter, mutate,
  rename, rename2, replace, stat2, and uniq.
• New command: csvtk filter2, filtering rows by artithmetic/string expressions like awk.