Add keep dropper

[FloopCZ]

I find very annoying to drop every column I temporarly create. It would be a nice feature to specify the columns to keep instead of dropping the useless ones.

E.g. I have columns a,b,c,d,e,f,g and want to keep only a,b:

// current
stream | drop<c,d,e,f,g>

// proposed
stream | keep<a,b>

This would be useful in the end of the streams on in case of reusing some stream parts (no idea what sources are provided)