Support tuple in by function

[TymonKilich]

Sorting by multiple columns using usual (non-natural) sorting works:

sort([["a1",  "a2"], ["a10", "a5"], ["a7", "a11"], ["a1", "a11"]], by = cols -> (cols[1], cols[2]))

and returns (sorted but not naturally):

4-element Array{Array{String,1},1}:
 ["a1", "a11"]
 ["a1", "a2"]
 ["a10", "a5"]
 ["a7", "a11"]

However, the same doesn't work for lt=natural

julia> using NaturalSort
julia> sort([["a1",  "a2"], ["a10", "a5"], ["a7", "a11"], ["a1", "a11"]], by = cols -> (cols[1], cols[2]), lt=natural)
ERROR: MethodError: no method matching natural(::Tuple{String,String}, ::Tuple{String,String})
Stacktrace:
 [1] (::Base.Order.var"#1#2"{typeof(natural),var"#21#22"})(::Array{String,1}, ::Array{String,1}) at ./ordering.jl:70
 [2] lt(::Base.Order.Lt{Base.Order.var"#1#2"{typeof(natural),var"#21#22"}}, ::Array{String,1}, ::Array{String,1}) at ./ordering.jl:53
 [3] sort! at ./sort.jl:480 [inlined]
 [4] sort!(::Array{Array{String,1},1}, ::Int64, ::Int64, ::Base.Sort.MergeSortAlg, ::Base.Order.Lt{Base.Order.var"#1#2"{typeof(natural),var"#21#22"}}, ::Array{Array{String,1},1}) at ./sort.jl:565
 [5] sort! at ./sort.jl:564 [inlined]
 [6] sort! at ./sort.jl:655 [inlined]
 [7] #sort!#7 at ./sort.jl:715 [inlined]
 [8] #sort#8 at ./sort.jl:763 [inlined]
 [9] top-level scope at REPL[15]:1

though it works with single argument (non-tuple) by function:

julia> sort([["a1",  "a2"], ["a10", "a5"], ["a7", "a11"], ["a1", "a11"]], by = cols -> cols[2], lt=natural)
4-element Array{Array{String,1},1}:
 ["a1", "a2"]
 ["a10", "a5"]
 ["a7", "a11"]
 ["a1", "a11"]

julia> sort([["a1",  "a2"], ["a10", "a5"], ["a7", "a11"], ["a1", "a11"]], by = cols -> cols[1], lt=natural)
4-element Array{Array{String,1},1}:
 ["a1", "a2"]
 ["a1", "a11"]
 ["a7", "a11"]
 ["a10", "a5"]