Add EditorConfig file

I have trouble submitting pull requests to this project because I have my editor configured to use spaces instead of tabs.  This `.editorconfig` file allows me to edit the code without worrying about re-configuring my editor.

More details on EditorConfig here: http://editorconfig.org/

A much longer example of a `.editorconfig` file: https://github.com/jquery/jquery/blob/master/.editorconfig