Add `findcols`

[bkamins]

What do you think about adding the following function:

findcols(predicate, df::AbstractDataFrame) = findall(predicate, eachcol(df))

Maybe such function would be more readable than using predicate.(eachcol(df)) for column selection?

[bkamins]

x-ref https://github.com/JuliaData/DataFrames.jl/issues/2508