--format csv leaves out duration

[winterlaite]

Could this please be added? For billing purposes start and stop times are not that important, but total time is. Right now it is 'start,end,note,sheet'. Could it please be 'start,end,duration,note,sheet'?
Thanks!

[samg]

It'd possible to do this by creating a custom formatter (as described at https://github.com/samg/timetrap#custom-formatters)