allow direct cd to non-existent target

```
mkdir somenewdir
z somenewdir
```

The above case will fail, and this patch fixes that case. A side-effect is that if you try to `z` to a dir which doesn't exist, then you'll get a cd error saying the dir does not exist.

it would be nice if there was a fallback to regular directory tab completion too, but I didn't focus on that.