`countmap` should accept any iterator

[pdeffebach]

julia> x = [1, 2, 3]
3-element Array{Int64,1}:
 1
 2
 3

julia> countmap(skipmissing(x))
ERROR: MethodError: no method matching countmap(::Base.SkipMissing{Array{Int64,1}})

[xiaodaigh]

countmap(skipmissing(x))

countmap(itr) = countmap(collect(itr)) should suffice although not efficient memory wise.

[nalimilan]

Actually addcounts! doesn't rely on indexing at all, so all what's needed is to remove the ::AbstractArray requirement in the signature (and add tests). The weighted method will require a few adjustments, but it should be quite easy too.

[alanedelman]

I could really use this.