support tmux 1.6

- Although #32 allow setting `capture_args`, it's awkward for people who use the same vimrc on different tmux versions. tmuxcomplete should default to `-J` only if supported (tmux 1.7+).
- tmux 1.6 `capture-pane` does not support -p (send to stdout). It sends the output to a tmux "buffer". So to work with this, tmuxcomplete can just echo each tmux buffer (and delete them afterwards, to avoid cluttering the user's tmux paste-buffers/history).