Parse Inf as float

[pearlzli]

I added this because I wrote a table containing Infs using CSVFiles but wasn't able to read it back in using the same.

julia> import TextParse: fromtype, tryparsenext, unwrap

julia> tryparsenext(fromtype(Float64), "Inf", 1, 3) |> unwrap
(Inf, 4)

julia> tryparsenext(fromtype(Float64), "inf", 1, 3) |> unwrap
(Inf, 4)

julia> tryparsenext(fromtype(Float64), "INF", 1, 3) |> unwrap
(Inf, 4)

julia> tryparsenext(fromtype(Float64), "-Inf", 1, 4) |> unwrap
(Inf, 5)