auto-changelog init does not produce a Prettier-compliant changelog

[mcmire]

Problem

• When creating a new changelog via auto-changelog init in a project that validates the changelog via auto-changelog validate --prettier, the validation step will fail, forcing the user to correct the changelog. This is annoying.

Suggested solutions

• The toString method on Changelog should produce a Prettier-compliant changelog.
• Prettier should always run after initializing, updating, or validating a changelog, and the --prettier option should be renamed to --no-prettier (or perhaps --disable-prettier).