Convenient type annotation for struct fields

I've been trying out Unitful lately, and I found myself wanting to build a `struct` whose members were `Quantity` objects. But I'd like to specify their type and units.

Using the `Quantity` type directly is a bit awkward:

```julia
struct State
altitude::Quantity{Float64, typeof(dimension(u"m")), typeof(u"m")}
end
```

One way I worked out was:

```julia
struct State
altitude::typeof(1.0u"m")
end
```

But I'd rather say `Float64` than `1.0`. So then I thought about creating a function to generate the `Quantity` type for me:

```julia
quantity(T::DataType, u::Unitful.Units) = Quantity{T, typeof(dimension(u)), typeof(u)}
struct State
altitude::quantity(Float64, u"m")
end
```

Is there a convenient way to do this that I am missing? If not, should something like this `quantity` function be added?