Added .editorconfig

Makes it easier for contributors. See [EditorConfig.org](http://editorconfig.org)
Please don't repeat the rules. We can put them together as:

```.editorconfig
[*]
charset = utf-8
end_of_line = lf
indent_style = space
indent_size = 2
insert_final_newline = true
trim_trailing_whitespace = true

[*.md]
trim_trailing_whitespace = false
```