Add C++ Support for Auto DevOps

Auto DevOps sounds extremely productive and I want to use it. Unfortunately, C++ isn't supported, which is strange considering the popularity of C++. Manual DevOps looks too tedious to set up, and I also don't like the idea of putting .gitlab-ci.yml files in the root directory of all my projects (and one for GitHub and one for Travis and one for Gerrit and.... you get the idea).

CMake is the de-facto build system for C++, and it uses a standardized filename that can be easily detected: CMakeLists.txt -- so adding C++ support via CMake support might be the easiest way to get C++ Auto DevOps (later, maybe support AutoTools? but meh AutoTools are 1990s technology so who cares if they're not supported [initially|ever]).