Extended opencl-platform option
In v2.8 option `opencl-platform` extended by 3 new values `"amd"`, `"nvidia"` and `"intel"` (case insensitive) it allow miner automatically find suitable platform index. Old behavior is still available, you can specify platform index by number. All changes are backward compatible.

In addition auto-configuration now available for NVIDIA and Intel if you specify an appropriate platform.