Update iTerm script

iTerm2 made some changes to its scripting, which causes 'got' to silently fail when using iTerm2, and setting
let g:gtfo#terminals = { 'mac' : 'iterm' }

Here is the new scripting commands:
https://github.com/gnachman/iTerm2/blob/master/iTerm2.sdef

Perhaps it might be necessary to check and have separate scripts  for iTerm/iTerm2.
Thoughts?
Oops, that will break on the next invocation (because `s:termpath` is no longer empty). Instead, it can just be:

```vim
     if (s:empty(s:termpath) && $TERM_PROGRAM ==? 'iTerm.app') || s:termpath ==? "iterm"
      silent call system("open -a iTerm '".l:dir."'")
```