clipboard: Add support for several more clipboards

This implements essentially the same heuristic as neovim, with the additional
(existing) special support for Cygwin.
See: https://github.com/neovim/neovim/blob/e682d799fa3cf2e80a02d00c6ea874599d58f0e7/runtime/autoload/provider/clipboard.vim#L55-L121
  - pbcopy, pbpaste (macOS)
  - cygwin (Windows running Cygwin)
  - wl-copy, wl-paste (if $WAYLAND_DISPLAY is set)
  - xclip (if $DISPLAY is set)
  - xsel (if $DISPLAY is set)
  - lemonade (for SSH) https://github.com/pocke/lemonade
  - doitclient (for SSH) http://www.chiark.greenend.org.uk/~sgtatham/doit/
  - win32yank (Windows)
  - tmux (if $TMUX is set)