Global support for `--prompt`

For those of us who can't be bothered to remember what arguments are required (and optional) for what commands, it would be nice to have a `--prompt` argument.

Usage could be like:

```
User: wp core config --prompt<CR>
WP-CLI: (1/3) --dbname=
User: some_db_name<CR>
WP-CLI: (2/3) --dbuser=
User: some_db_user<CR>
WP-CLI: (3/3) --dbpass=
User: <CR>
WP-CLI: Success: wp-config.php file created.
```

`--prompt` is semantically different than `--interactive` in that the former provides an interface for entering command arguments, and the latter implies interactivity within the command. See #635.

Related #600, #621, #540