Clang code model: allow users to pass their own options

I think a very handy feature would be to allow users to pass their own options to the clang parser (per project). This way it would be to possible to apply new or rare clang options without having to support them explicitly through the GUI.

Personally, I would use this to pass "-std=c99" to clang, since the default is C++ at the moment, which leads to clang warnings with C99 code. Of cause the proper fix would be to pass the right options according to the file type, but since proper C support is obviously not high on the priority list, this would be a good workaround.