CMake: Allow to skip install

[feliwir]

There should be an option like WRENBIND17_SKIP_INSTALL - when using CMake libraries as a submodule it's not always favorable to have install to be run

[matusnovak]

Hi @feliwir

Yes that's very reasonable. Would it be fine with you if such option would be ON by default? In order not to break any existing usage of this library.