Add CMake functionality for plugins to be able to copy themselves during build

Inspired by some well-meaning CMake code in https://github.com/OSVR/OSVR-Leap-Motion -

For testing purposes, it's useful to have the server be able to find the plugin you just built. Since there's no nice plugin search path setup yet, we could add capabilities in the CMake scripts to optionally copy the built plugin into the OSVR tree, presumably as a separate target.
