Add '--single-attribute-per-line' option to Playground

[apmeehan]

Description

Just adds the new --single-attribute-per-line option to the Playground.

Checklist

• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory).
• (If the change is user-facing) I’ve added my changes to changelog_unreleased/*/XXXX.md file following changelog_unreleased/TEMPLATE.md.
  (I presumed this step wouldn't be necessary for such a minor update as this — apologies if I'm mistaken)
• I’ve read the contributing guidelines.

✨Try the playground for this PR✨

[apmeehan]

Sorry I'm not familiar enough with those Netlify tests to know why they might have failed, or how to resolve.

[fisker]

Thank you!

[apmeehan]

@fisker You’re welcome! Glad to be able contribute any way I can to a lib that makes my life easier nearly every day, and thank you for all your work maintaining it!