minor change in vcat implementation

[bkamins]

Changes in this PR:

• make reduce(vcat, dfs; cols) behave in the same way as vcat(dfs...; cols) (this means: disregard data frames with 0 columns and acccept cols keyword argument)
• make reduce(vcat, dfs; cols) also accept a tuple of AbstractDataFrames and not only an AbstractVector of them

[bkamins]

Thank you. This PR can wait till the next release, but if we need to do 0.18.2 I would include it, as in my opinion it is a patch making sure vcat and `reduce(vcat,...) are working consistently (as they should).

[nalimilan]

As you like.

[pdeffebach]

Yeah thanks for this! I didn't know it was allowed to add a keyword argument when dispatching reduce on a function.

[bkamins]

Actually the issue of dispatch with kwargs is quite tricky (I recently had another issue in DataFrames.jl related to this; here is a status of a long unresolved issue that should improve dispatch rules of functions taking kwargs JuliaLang/julia#9498) but in this case it should be safe.