cbimport: should add flag of fields to ignore

I think that should add a flag for keys we want to ignore in the import process.
For example:
I've exported a file from bucket X to a json file, using the include-key flag.
 
./cbexport json -c localhost -u Administrator -p password -b default -f lines -o /home/default.json -t 4 --include-key docKey
 
Now when I import it back - I don't want to have an extra field in the document named docKey. I wish to use it only of generating the doc ID but not more.
 
It also useful when interacting with external systems which embed the key into the data file.