tmux: Open new/split planes with current pane's path
tmux: Open new/split planes with current pane's path

Based on a commit by @sds at:

  https://github.com/sds/dot/commit/787b4061737725d

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent