neovim support for mac via VimR

VimR nightly builds are for all intents and purposes macvim with neovim, while neovim-gnome-terminal-wrapper might work for linux (not a linux user).

For now I'm using a if has('nvim') in my .vimrc for macvim compatibility, but it would be nice to get rid of that stuff.

Not opposed to this at all. Would happily accept a pull request. I think adding an option that allows you to override the editor would be great.