Add vim undo history support

[shrop]

Per @tlattimore's awesome post: http://shelldevel.net/post/112278835197/better-undo-history-in-vim

Need to setup a bit in script/bootstrap to check if .vim/history exists. If not, create it.

[tlattimore]

I actually haven't added this to my dotfiles yet. So I look forward to seeing how you do it.