`list --field=` ignores `--format=json`

```
$ wp theme list --field=name --format=json
```

Current:

```
twentyeleven
twentyfourteen
twentyten
twentythirteen
twentytwelve
```

Expected:

```
["twentyeleven", "twentyfourteen", "twentyten", "twentythirteen", "twentytwelve"]
```

Also, the prefix logic should work for `--fields=` too: `--fields=title,content`