csvtk v0.8.0

Changelog

• new command csvtk gather for gathering columns into key-value pairs.
• csvtk sort: support sorting by user-defined order.
• fix bug of unselecting field: wrongly reporting error of fields not existing.
  affected commands: cut, filter, fitler2, freq, grep, inter, mutate,
  rename, rename2, replace, stats2, uniq.
• update help message of flag -F/--fuzzy-fields.
• update help message of global flag -t, which overrides both -d and -D.
  If you want other delimiter for tabular input, use -t $'\t' -D "delimiter".