Option to omit header row in CSV export

[simonw]

?_header=off - for symmetry with existing option ?_nl=on.

[simonw]

Can work on this as part of #1062.

[simonw]

Inspiration was this script: https://gist.github.com/simonw/f6e3cd29fde5d15ea9cd746c942046ba - which pipes output through tail -n +2 to strip off the headers.