Optionally autosave when switching out of vim

Optionally (if g:tmux_navigator_save_on_switch defined) save the current file when switching out of vim.

I really like the option to autosave(like on the the gVim's 'focus lost' event).

Typically with these checks I go with both existence and truthiness check:

``` vim
if exists('g:tmux_navigator_save_on_switch') && 'g:tmux_navigator_save_on_switch
```