[fzf-tmux] Add option to start fzf in tmux popup window

Requires latest tmux built from source (e.g. brew install tmux --HEAD)

Examples:

  # 50%/50% width and height on the center of the screen
  fzf-tmux -p

  # 80%/80%
  fzf-tmux -p80%

  # 80%/40%
  fzf-tmux -p80%,40%

  # Separate -w and -h
  fzf-tmux -w80% -h40%

  # 80%/40% at position (0, 0)
  fzf-tmux -w80% -h40% -x0 -y0

You can configure key bindings and fuzzy completion to open in tmux
popup window like so:

  FZF_TMUX_OPTS='-p 80%'