Add .editorconfig

To improve codebase maintainability and consistency, let's use and `.editorconfig` file containing the coding conventions settings ([documentation here](http://editorconfig.org/)).

Note that not all IDEs/text editors support `.editorconfig` files out of the box, you may have to download a specific plugin to make it work.

![woah](http://i.imgur.com/P1URP.gif)