add persistent undo to vim

For vim 7.3 and up. Should find a way to detect versions

RUNME

@@ -6,6 +6,7 @@
 # Vim
 ln -s dotFiles/vim/ ~/.vim
 ln -s dotFiles/vim/vimrc ~/.vimrc
+mkdir ~/.vimundo
 
 # Zsh
 ln -s dotFiles/zsh/zshrc ~/.zshrc

vim/vimrc

@@ -26,6 +26,10 @@ set splitbelow "New splits appear below current window instead of above
 set splitright "New splits appear right of current window
 set ttyfast "Smooth movement
 
+" Persistent undo
+set undofile
+set undodir=~/.vimundo
+
 "if version >= 700
 "    set mouse=a "mouse support for 7.x, but don't use this if we use screen
 "    because it has no effect, and then just becomes annoying for copy/paste