Set undodir to a more appropriate location

Also, make sure that appropriate location exists :)

Per a conversation with tpope, here:

   https://github.com/hashrocket/dotmatrix/commit/6d0886aa81b897b3b4e656b72615bcf79428e314

I'm setting the undodir to an opt-in location. So that that opt-in
location is there by default for dotmatrix installs, I've added a
routine to create that directory if it's not present when you
install/upgrade.