quantity(FloatType, unit) function

Declaring a Unitful unit in a struct could be made nicer, if the following function **quantity(..)** is provided in Unitful:

```
using Unitful

# The following function should be defined inside Unitful
quantity(FloatType, unit) = Quantity{FloatType, dimension(unit), typeof(unit)}

mutable struct Data{FloatType <: AbstractFloat}
length::quantity(FloatType, u"m")
end

L = Data{Float64}(2.0u"mm")
@show L
```

Of course, it is easy to introduce such a function in an own package. However, it would be better if this feature is directly provided by Unitful.

Start by reading Unitful's existing quantity and unit type definitions, then compare them with the proposed Julia example. The work is done when Unitful provides the requested quantity(FloatType, unit) form and the shown Data struct can declare and construct its length field as demonstrated.

Written by the indexing model from the issue text.