Prefer $_ for more terse set up instructions

Open to discussion, the `$_` variable is set to last argument of previous command executed. Ergo,

```bash
mkdir foo && cd $_
```
is the same as 
```bash
mkdir foo && cd foo
```

I'm not sure if this adds value as it may make setup harder to follow, but it's something I've been using lately.  Thoughts @dcalhoun ?