Add clang-format configuration

The library lacks a guide about how the code should be formatted.
There are inconsistencies and some lines contain too many characters to fit on a screen.

A [clang-format](https://clang.llvm.org/docs/ClangFormat.html) configuration with a column limit is crucial for both consistency and readability.

It could be discussed that whether the formatting should be enforced by continuous integration. A problem with enforcing the formatting is the variance in the layout between different clang-format versions (see: http://lists.llvm.org/pipermail/cfe-dev/2017-June/054351.html). For example, if I format the codebase with clang-format version 11 and then verify it with version 10 on the CI server it emits errors. So, if we were to strictly enforce the formatting, we would have to install the same clang-format version everywhere.