Implement code formatting on save

[AlexGustafsson]

It would be nice to have code formatting before saving (or at least the ability to format the code via a command). On macOS clang-format comes installed alongside clangd when installing llvm through brew. To my knowledge it does on Ubuntu (and I'm sure it does on other distributions) as well via the package clang.

I guess the plugin could decide to use a default style such as llvm or google if none is defined in a .clang-format file.