~/.vimrc: Enable persistent undo.

From tpope's [vim-sensible](https://github.com/tpope/vim-sensible).

.vimrc

@@ -35,6 +35,8 @@ set hidden
" Oh YEAY, finally. Store them (swaps) all in *one* directory.
set directory=~/.vim/tmp/
set undodir=~/.vim/tmp/
set undofile
" Wrap lines in textfiles automatically.
" (Any way to enable in comments?)