Change rename.vim to Tim Pope's vim-eunuch

Does what rename.vim does (`:Move` or `:Rename`) **plus**:

* Adds `:Unlink` or `:Remove` to delete the current buffer + file
* Adds `:Mkdir` (with no argument, create the current file's containing
  directory)
* Adds `:SudoWrite` if you forget to edit a file as root
* Automatically chmods a file to `+x` if it starts with `#!`
* Tim Pope! :heart:
Force vertical diffs

Fugitive was updated to switch to horizontal diffs on narrow screens.
Everyone I've seen experience this behavior finds it disorienting. This
setting forces a vertical diff without users having to use different
shortcuts to enter diff mode.