VIM: Add my prefered colourscheme: gruvbox and set leaders

This involved adding a submodule which contains the gruvbox plugin.

Normally I have the background transparent when using vim but I am going
to try without that.

I have set my leader key to space and my local leader to \ which I am
used to

nvim/.config/nvim/init.vim

@@ -0,0 +1,2 @@
+let mapleader = "\<Space>"
+let maplocalleader = "\\"