.SDcols=function shorthand

[MichaelChirico]

Much like .SDcols = patterns is useful as a shorthand for filtering columns and much like setnames now accepts functions, .SDcols=function is (1) currently an error and (2) should translate pretty naturally as a shorthand for filtering in .SDcols.

e.g.

DT[ , lapply(.SD, sum), .SDcols = is.numeric]

Should be easy to apply ! as well.