[feature request] Extend `seqgen` use to a file

Hi, it would be great, starting from input.csv

fieldA,fieldB
a,1
e,13
e,7

to be able to run something like

mlr --csv seqgen --step 2 --start 1 -f newfield input.csv

and have

fieldA,fieldB,newfield
a,1,1
e,13,
e,7,3

without specifying --stop and use as stop the number of records.

Thank you