Project-wide EditorConfig support

[ljakab]

I'd say that it's pretty non-controversial that consistently formatted code makes reading it and contributing to it easier and more pleasurable. To avoid code reviews being about white spaces and formatting in general, and make it easier for new contributors to write properly formatted code effortlessly, without hunting for requirements in documentation and then setting up their environment, we should support EditorConfig and add the appropriate .editorconfig file to each of our repositories.