support IO for writetable

[hhaensel]

This PR should fix #78

[hhaensel]

I tried reading directly from io for the runtests.jl, but that failed. Maybe that's for another issue ...?

EDIT: I was stupid. I forgot to reset the io buffer before testing. So no new issue...
... and a nicer test routine

[hhaensel]

While this is not in place, I helped myself with the following workaround

function XLSX.writetable(io::IO, tables::Union{Vector, Tuple}; kwargs...)
    XLSX.openxlsx(io, mode="w") do xf
        for (i, table) in enumerate(tables)
            table isa Pair || (table = nothing => table)
            if i == 1
                table[1] !== nothing && XLSX.rename!(xf[1], table[1])
            else
                table[1] === nothing ? XLSX.addsheet!(xf) : XLSX.addsheet!(xf, table[1])
            end
            XLSX.writetable!(xf[i], table[2]; kwargs...)
        end
    end
end

XLSX.writetable(io, tables; kwargs...) = XLSX.writetable(io, [tables]; kwargs...)
XLSX.writetable(io, tables...; kwargs...) = XLSX.writetable(io, tables; kwargs...)

[felipenoris]

Looks good. Thanks!