Sugggestion: make `z` and `z -` work like `cd` and `cd -`

[Arnie97]

I'm expecting z to work like a superset of cd, otherwise I have to fight against my muscle memory to use z in some cases and cd in others.

If no argument is specified, cd will change the current directory to $HOME; and if argument is -, cd will change to the previous directory. In both cases, z now works like z -l.