Write .clang-format for C++ style

[davidzchen]

Related to #142, add a .clang-format that codifies the C++ style for Jsonnet.

[sparkprime]

I'd be OK with minimally changing the style if .clang-format is not able to enforce the way it currently is, so we should do this first, and then document the resulting state.

Presumably, it'll be possible to do run the C++ reformatter over the whole code base and update it, if that is the case?

[davidzchen]

Sounds good to me. We can script clang-format to automatically reformat all the C++ files in the codebase using the style defined in the .clang-format file.

Once we have this done, we an also use clang-tidy to run lint checks, similar to what we do internally. :)