Integrate WP-CLI with Singer.io

[schlessera]

It would be extremely useful to have WP-CLI entities be both taps as well as targets for https://www.singer.io/, the open-source ETL.

For taps, we could probably add a --format=tap that lets you transform all available WP-CLI lists into a valid tap.

For targets, we might be able to add a flag for some commands to accept input from taps or even auto-detect singer taps.