Environment variable to choose device given multiple backends

Suppose one has two backends, say `CUDA` and `OPENCL`, then there is currently no way to choose a device of a specific backend using an environment variable, like the ones listed in http://arrayfire.org/docs/configuring_environment.htm

It'd be great if we have an option like `AF_DEFAULT_DEVICE=OPENCL_1` which would set `device=1` with the `OPENCL` backend.