Command Buffers?

I suggest to add command buffer when executing programs...
```cpp
   auto queue = device.queues().at(0);
   auto cmd = device.command(); // something alike this
   auto program = vuh::Program<Specs, Params>(device, "saxpy.spv"); // load shader
   program.grid(128/64).spec(64)(cmd, {128, 0.1}, d_y, d_x); // write to command
   queue.submit(cmd); // submit command 
```
