Handle new scope names for C++

In the next release of Atom (v0.166), the scope names for C++ and Objective-C++ will change from `source.c++` and `source.objc++` to `source.cpp` and `source.objcpp`. This is being done to work around some issues with CSS classes containing '+' characters. This PR adds handling for the new scope names, but will continue to handle the old ones, for compatibility with old versions of Atom.

Refs atom/language-c#54.