Add support for compiling single file when using CMake project

When using qmake based projects it is possible to build only single file from project.

This is a very useful feature and it would be great if that support would be extended to CMake based projects.

For example, if using Ninja generator, the build for single file should invoke `ninja CMakeFiles/project.dir/path/to/source/file.cpp.o`.

When using Clang Code model, clang backend actually performs something very similar and displays compile issues in real time, so adding this option to actually perform compilation should not be a difficult to implement. Am I wrong?