`wp core` should work without a WP install

I ran `wp core` and saw:

```
hmn-vagrant ➜  wp-cli  wp core
Error: This does not seem to be a WordPress install.
Pass --path=`path/to/wordpress` or run `wp core download`.
```

I expected to see the `wp core` commands I could run without WP installed.

Same thing for #602 — it would be nice to see the commands that could be run without WP installed.