Introduce `--field` option for `get` subcommands

Currently, we have `wp post get --format=content`, which is kind of a hack.

`wp post get --fields=content` would still have to return a table or a JSON object, to be consistent with all the other commands that have a `--fields` option.

`wp post get --field=content` would only return the raw value.

And, obviously, you could use it with any field: `wp post get --field=title`, `wp post get --field=status`