Narrow column type

[cmcaine]

Related to #64

If you read a csv with a column that is initially empty, TextParse.jl assigns that column a type of Any. I propose that the column type should either be narrowed to a union of the actual types used or that the initial type should be Missing and it should be promoted as in #64.

Complete example

IOBuffer("""
floatcol,mostlymissingcol
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,
-1.,10.
""") |> TextParse.csvread |> typeof

# Returns
# Tuple{Tuple{Array{Float64,1},Array{Any,1}},Array{String,1}}