Add .editorconfig file for formatting specification

Add `.editorconfig` to specify formatting style of the project, for example:

```ini
root = true

[*]
charset = utf-8
end_of_line = lf
insert_final_newline = true
trim_trailing_whitespace = true

[*.{c,h}]
indent_style = tab
indent_size = 8
tab_width = 8
```

This should also improve formatting in GitHub web editor.