Added hunter_cmake_args function.

This allows a project's hunter.cmake file to reuse one of the standard CMake schemes, but still customise the CMAKE_ARGS used without polluting the config.cmake file.
I think logic here must be not so simple, i.e. if we have `CMAKE_ARGS` in both global `<Project>/hunter.cmake` and local `config.cmake` then we need to merge them. And if there are some common variables => local `CMAKE_ARGS` should have a higher priority. May be we can leave this simple behaviour for now, but in this case please open the `enhancement` but to not to forget about it :)