Prefix command line arguments with --vtkm-

Currently, the VTK-m arguments parsed in vtkm::cont::Initialize have names like --device or -d that do not necessarily distinguish them from arguments from other potential applications. This can be a problem if argument names overlap.

To solve this, we will prefix all VTK-m command line arguments with --vtkm- to differentiate them from possible application command line arguments. This will allow an application to call vtkm::cont::Initialize, which will remove any VTK-m-specific argument without worrying about removing an argument the application expects. It also makes it easier for applications to detect VTK-m arguments and ignore them.