Using `//` comments for C++ header files

Hello :)

Currently `.h` files use `/*` style comments, and I'd like to contribute a way to use `//` comments instead. I presume just changing the default is not an option, so I'd like to ask what would be the most principled solution. A command line flag?

The issue names no files or tests. Start by examining addlicense's command-line entry point and its handling of .h files, then determine how comment style is currently selected. Done should be an agreed, documented mechanism for using // comments without changing the default.

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