Introduce `--field` option for `list` subcommands

We introduced a `--field` argument for `get` subcommands in #776. We could also add it for `list` subcommands:

```
wp site list --field=url
```

would be equivalent to:

```
wp site list --fields=url --format=csv | tail -n +2
```

and would be shorter than the proposed 'column' format:

```
wp site list --fields=url --format=column
```

Depends on #779

TODO:

* [x] `post list`
* [x] `user list`
* [x] `comment list`
* [x] `site list`
* [x] `plugin list`
* [x] `theme list`