Allow reading values from STDIN

Pass value inline (current):

```
$ wp option set key value
```

Pass value from a file:

```
$ wp option set key --format=json < value.json
```

Pass a multiline value interactively:

```
$ wp option set haiku<CR>
From time to time<CR>
The clouds give rest<CR>
To the moon-beholders.<Ctrl-D>
```

I'm thinking this would also work for `wp option add` and all the `-meta` commands.

Tasks:

* [x] implement for `option add` and `option update`
* [x] implement for `*-meta add` and `*-meta update`