Add .clang-format file

[eschnett]

I like using clang-format to format source files. This takes much of the tedium out of editing C++ files.

Would you consider adding a .clang-format file to the repo? This would define your preferred indentation style.

Ideally, this would be used for all source files. To avoid churn, one could begin to use this only to format newly added files, or newly added functions.

[franzpoeschel]

I've started experimenting with this on #1032.

[ax3l]

Sounds good to me, let's iterate in the PR.

[ax3l]

Found today: CMake targets
https://github.com/TheLartians/Format.cmake