Automated formatting

[jgopel]

Would there be any interest in introducing a .clang-format file for this library? It would not set precedent for boost - gil, graph, histogram, multiprecision, nowide, outcome, and yap all currently have .clang-format files. As far as I can tell, those are each unique format files, so I think there would be large latitude to use any configuration that makes sense for this library.