Sync the behavior of push!, vcat and append! in DataFrames.jl with Base

[bkamins]

When the decision in JuliaLang/julia#25904 is made I think we should use consistent rules by default (and optionally have a keyword argument to add/reduce flexibility - depending if JuliaLang/julia#25904 will end up being flexible or strict).

[bkamins]

@nalimilan - actually looking at the current API the only change I would make is to add :identical option for cols kwarg, that is even stricter than :equal and requires not only that the names match but that they must be in the same order. Then we could, in the future change vcat to default to :identical instead of current :equal.

[nalimilan]

Why not. Names aren't very descriptive but it's hard to find better ideas.

[bkamins]

Yes - I had a problem with :equal as it is already in use, but I could not come up with anything better.

[nalimilan]

We could deprecate :equal if we can find a better name...