Please add argument to use only NVIDIA/AMD gpus

[heartoflusho]

Please add an argument to only use only one brand of GPU. It would be really helpful, and I would use Gminer on more rigs if hardware crashes reordering the the GPUs didn't risk significant downtime on rigs with two miners running.

[develsoftware]

add --opencl 0 to skip AMD devices

[heartoflusho]

Thanks