Switch `prettier` to an optional- or peerDependency

[sirlancelot]

With the release of Prettier v2.x, projects wishing to update to the latest version will download both versions of Prettier due to this module having a hard dependency on v1.x.

Since Prettier is only require'd when prettify == true and NODE_ENV != "production", can this dependency be switched to an optionalDependency or peerDependency? Or perhaps its version requirement relaxed to include v2.x?

[P4sca1]

Totally agree with making prettier a peerDependency. Currently this causes problems in my workspace, because yarn keeps placing a version < 2.0.0 in the .bin folder, even though I have listed prettier 2.0.4 in my dependencies.

[haoqunjiang]

• Making it a peer dependency ❌ This would a breaking change
• Relaxing the version requirement to 2.x ❌ This would break Node.js 8.x users
• Making it an optional dependency ✅ This would be mostly fine

---

Currently this causes problems in my workspace

To be fair, it is Yarn that causes the problem, not this package.