Add .editorconfig file. 

Hi, 
This file is for unifying the coding style for different editors and IDEs.
More information at http://EditorConfig.org
Hm... well, if I'm interpreting the docs correctly, we shouldn't specificy tab_width nor indent_size?

>For example, tab_width need not be specified unless it differs from the value of indent_size. Also, when indent_style is set to tab, it may be desirable to leave indent_size unspecified so readers may view the file using their preferred indentation format.