Implement PR comments with changelog in body

[jpsullivan]

Essentially the same as https://github.com/pmowrer/semantic-release-github-pr but with some modifications:

• Allow additional text to be added under the changelog details, such as providing short-lived deployment URLs
• Add toggle to prevent changelog details from being added to comment
• Global toggle
• Support multiple baseBranch matchings (i.e. ['master', 'develop'])