Add parameters to support CUDA or OpenCL

[doterax]

I have been continuing developing Guetzli with CUDA and OpenCL support. And it's cool to support this additional parameters in your project.

--c optimized CPU version
--cuda use CUDA
--opencl use OpenCL

https://github.com/doterax/guetzli-cuda-opencl

[tanrax]

If I perceive a demand to include it, I will gladly do so. And if there is a pull request, I will accept it.
For the time being I will leave this issue open as a suggestion.