Add support for --prompt option for wp core config

As per @danielbachhuber's #673, adding a proposal for `--prompt` support for `wp core config`.

It's reusing `cli\input` as one of the cli-tools functions as @scribu mentioned [here](https://github.com/wp-cli/wp-cli/pull/600). Not sure about the right fallback as the Synopsis validation had to be updated to support the --prompt in the first place.