Also set tmux navigation flag on FocusGained

Make using the `@vim$TMUX_PANE` window flag much more robust by
unsetting it every time the zsh prompt is drawn and setting it again
using a `FocusGained` `autocmd` in vim. This removes the need for the
hacky check for zsh in the conditional defined in the `in_vim` variable.

autoload/tmux.vim

@@ -0,0 +1,9 @@
+let s:tmux_option = '@vim'.substitute($TMUX_PANE, '%', '\\%', 'g')
+
+function! tmux#setNavigationFlag() abort
+  call system('tmux set-window-option '.s:tmux_option.' 1')
+endfunction
+
+function! tmux#unsetNavigationFlag() abort
+  call system('tmux set-window-option -u '.s:tmux_option)
+endfunction