update NuGet packages including assembly references

Use it like this in order to update all packages in a solution:

```
Target "UpdatePackages" (fun _ ->
    !! "*.sln"
    |> Seq.iter (NugetUpdate id)
)
```

Finer-grained control is available over the `setParams` function.

Any ideas how to reduce boilerplate with the parameters structs, default values and command line builders?