COLUMNS overrides fd width
COLUMNS overrides fd width

* guesswidth_unix.go: Check the env to see if the fd width should be
  overridden with the COLUMNS var

Signed-off-by: Jeff Mickey <5c2dd944dde9e08881bef0894fe7b22a5c9c4b06@codemac.net>

alecthomas/kingpin,wwalker/kingpin