Fix: no more .file.un~

When using set undofile Vim creates .file.un~ files. It does this so you can keep
using undo on a file even though you restarted or closed Vim between
your actions. I don't like these .file.un~ all over the place so I use
set undodir in .vimrc to define one directory to keep all these files.
The installer creates the path for this directory during install. At the
moment the directorypath is: ~/.vim/persistent-undo/