Add env var $CMD

$_ is nice to get the program name.
But there's no way to get the entire command line string(maybe there's a way I don't know, please tell me if I am wrong). So I add a readonly env var $CMD, and it is very useful in function fish_title.
```fish
function fish_title
  if [ $_ = 'fish' ]
    echo (prompt_pwd)
  else if [ $_ = 'ssh' ]
    echo $CMD
  else
    echo $_
  end
end
```
I'm not sure if CMD is a good name or not, because $_ is a really very cool name.

Thanks.