Strip vim entirely; EDITOR is nano

dissimulo does not use vim. Removes the editor, its config tree, its plugin
manager, and everything that would reinstall it on a re-bootstrap.

Deleted (82 files):
  vim/ (64)          the whole config tree, 1.7M
  doc/vim/ (8)       its documentation
  .vimrc, vimrc, .vim
  bin/yadr/vundle.rb, yadr-vim-{add,delete,list}-plugin
  bin/fix_macvim_external_display.sh   MacVim on macOS, dead here anyway
  vimify/            vi keybindings for readline/libedit, dropped on request

Edited:
  Rakefile        drop the vundle require, the vim install block, the
                  vundle_migration and install_vundle tasks, their two helper
                  methods, and the macvim homebrew line. Without this a
                  re-bootstrap would pull all of it straight back.
  install.sh      .vim/.vimrc out of the symlink list
  zsh/vi-mode.zsh EDITOR/VISUAL now nano. The zsh vi keybindings stay; they
                  are not vim.
  zsh/aliases.zsh yav/ydv/ylv, the MacVim detection block and ve removed;
                  ae/ze/gi now open nano
  git/gitconfig   mvimdiff mergetool removed, editor set to nano
  .tmux.conf      vim-tmux-navigator replaced with direct select-pane binds.
                  It ran ps piped to grep on EVERY C-h/j/k/l press to ask
                  whether vim held the pane: two forks per pane switch for a
                  question that can now only answer no. mode-keys and
                  status-keys vi stay, being tmux own key style, unrelated to
                  the editor.
  .gitignore/.dockerignore/Dockerfile   dead paths and the vim package

Also renames the installer banner to DRUNKENDOTFILES, piped through lolcat
when present with a plain-text fallback.

Left alone: the .yadr paths (renaming the checkout is a separate job), generic
swap-file ignores, the fasd viminfo support (third-party), and prose mentions.

git/gitconfig

@@ -98,13 +98,9 @@
   pretty = format:%C(blue)%ad%Creset %C(yellow)%h%C(green)%d%Creset %C(blue)%s %C(magenta) [%an]%Creset
 [mergetool]
   prompt = false
-[mergetool "mvimdiff"]
-  cmd="mvim -c 'Gdiff' $MERGED"     # use fugitive.vim for 3-way merge
-  keepbackup=false
 [merge]
   summary = true
   verbosity = 1
-  tool = mvimdiff
 [apply]
   whitespace = nowarn
 [branch]
@@ -115,7 +111,7 @@
   default = upstream
 [core]
   autocrlf = false
-  editor = vim
+  editor = nano
   excludesfile = ~/.yadr/git/gitignore
 [advice]
   statusHints = false