Add editorconfig

[thaugdahl]

Self-explanatory PR :)

[mscuttari]

I'm not contrary to the addition of .editorconfig, because ClangFormat indeed does not to go beyond C / C++ source files. However:

1. Having a capture-all section like the one present in the proposed file would lead to a duplicate configuration to be maintained coherent with respect to the LLVM styling guidelines.
2. Some parts of the code are explicitly marked as to be ignored through the clang-format on/off comments. Some IDEs would ignore the ClangFormat entirely if an .editorconfig file is found, thus linting the code in an undesired way.

For these reasons I would prefer only the Python, CMake, and Makefile configurations to be there, letting ClangFormat to be the choice for C / C++ files.

By the way, it is not super clear to me why the Makefile configuration requests for tabs instead of whitespaces, like the rest of the project, but this may just be something beyond my knowledge.

[mscuttari]

/fast-forward