Extrapolate to missing

It would be nice to support extrapolating to `missing`

```julia
> using Interpolations, Missings
> A = rand(20)
> A_x = collect(1.0:2.0:40.0)
> knots = (A_x,)
> itp = extrapolate(interpolate(knots, A, Gridded(Linear())), missing)
> itp[42]

MethodError: Cannot `convert` an object of type Missings.Missing to an object of type Float64
This may have arisen from a call to the constructor Float64(...),
since type constructors fall back to convert methods.

Stacktrace:
[1] getindex(::Interpolations.FilledExtrapolation{Union{Float64, Missings.Missing},1,Interpolations.GriddedInterpolation{Float64,1,Float64,Interpolations.Gridded{Interpolations.Linear},Tuple{Array{Float64,1}},0},Interpolations.Gridded{Interpolations.Linear},Interpolations.OnGrid,Missings.Missing}, ::Int64) at /home/cedric/.julia/v0.6/Interpolations/src/extrapolation/filled.jl:25
[2] include_string(::String, ::String) at ./loading.jl:522
```