Add strict kwarg to names!
Add strict kwarg to names!

Adds `strict` keyword argument that by default prevents setting duplicates in column names.
Additionally the proposed logic of creation of new `Index` is simplified as we can expect that if someone calls `names!` then most of the column names would change (small changes probably are better handled by `rename!`).
JuliaStats/DataFrames.jl,scls19fr/DataFrames.jl,JuliaPackageMirrors/DataFrames.jl,campanja-forks/DataFrames.jl