Add --columns=<col_list> flag to .nullify

[curlette]

This ticket is based on a student question from PPAML summer school.

If this were implemented, then I think there would still be a tradeoff between using SQL and BQL for nullifying values.
In SQL, you could set multiple values in a single column to NULL simultaneously:

%sql UPDATE <table> SET <col> = NULL WHERE <col> IN (<values>)

whereas with BQL you could set a single value to null for multiple columns simultaneously:

%bql .nullify --columns=col1,col2,col3 <value>

However, if .nullify could also take multiple values, it would be the more efficient choice.