C++ equivalent of `allow_headerless` synchronization

[akgoins]

I know that you can set allow_headerless = True in python, but is there an equivalent option for C++? Are there plans to add it in the future?

[clalancette]

It looks like there is nothing equivalent in C++ right now. Certainly we would be happy to review a pull request that added it to the C++ side of things, so please consider contributing.