Allow invocation of external commands on specific column

given that the program "decorate" exists, accepting 2 args - type of data to decorate and key, example being decorate day_of_week 1399248007040, it would ROCK if I could write the following:

jku -t '[_.timestamp, decorate(day_of_week, _.timestamp)]'

and get the output:

1399248007040\tMonday

if the shell command returns non-zero exit code, jku should abort
