Add .editorconfig file

As discussed in this ember [issue](https://github.com/emberjs/ember.js/pull/5200), consistent coding style is useful. Adding this file for the ember-cli project itself for now, if it is found to be useful, I can add one to the app blueprints as well. Ember-cli is an opinionated tool that is built around the best practices, I think this is a suitable addition to the project. For more information about EditorConfig one can checkout the [website](http://editorconfig.org/).

@stefanpenner @rwjblue can you please review this?