Add current dir with new shells

tmux v1.9 [removed][1] the `default-path` option. To recreate this
feature we add them as key bindings as descibed in [this SO answer][2].

[1]: https://github.com/ThomasAdam/tmux/commit/4538c269d0b366a770a5a5ebfe0c5007569edbc1
[2]: http://unix.stackexchange.com/a/118381/8541