Scopes for C++ and Objective-C++ are changing

[maxbrunsfeld]

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. You may want to update your README with the new scopes. I thought I'd leave it to you rather than submit a PR because I'm not sure if you want to mention the old scope names for backward-compatibility purposes or not.

Thanks for maintaining this package!

Refs atom/language-c#54.