add editorconfig file for neovim

[EditorConfig](http://editorconfig.org/) is a cross-editor file which 
specifies a project indent style, newline, etc, etc. This is a minimal
"document the current standard" file which should be updated once #104 is
decided.

It seems somewhat perverse to include a cross-editor configuration file for
neovim when it is probably one of the few projects where one knows which
editor the developers will be using. That being said, vim (and by extension
neovim) has good support for EditorConfig.