[Feature request] Unitful compatibility

_Issue discovered by @AndiMD originally in [#295](https://github.com/JuliaMath/Polynomials.jl/issues/295#issuecomment-737641577):_

Possibly related, [Unitful.jl](https://github.com/PainterQubits/Unitful.jl) compatibility:
```julia
julia> using Polynomials, Unitful
julia> fit([1.0,2.0,3.0,4.0].*u"m", [6.0,4.0,7.0,7.5].*u"m",2)
ERROR: DimensionError ...
```

---

Response from @jverzani on 5th Dec., 2020:

> Yeah, [...] I'm not sure there is a proper fix. For fitting, a Vandermonde matrix is constructed with columns which would need units of u^0, u^1, ...,u^n. So currently there is an error as we type that matrix by eltype(x).
>
> Some discussion on the issue of using units with matrices can be found here:
[PainterQubits/Unitful.jl#46](https://github.com/PainterQubits/Unitful.jl/issues/46)
>
> with the simplest recommendation being to strip the units and add them back, as needed.