Add feature release_notes to generate release notes

[alexellis]

Description

This feature enables notes to be collected from all merged/closed PRs since the last release. The notes are then set/saved on the release body field.

Motivation and Context

Closes: #89

How Has This Been Tested?

Tested in a console application outside of Derek and e2e with Derek operating against the openfaas/org-tester repo.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Users must opt-in to use this feature, but it requests new permissions:

• events: release
• write to repository contents

User guide has been updated, to get this feature add release_notes under features: in .DEREK.yml

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[alexellis]

GitHub behaviour:

With Derek's feature enabled:

[alexellis]

Notifying ".CUSTOMERS"

moby - @thaJeztah PTAL @rgee0 Subsurface-divelog @neolit123 @dirkhh PTAL @ewilde PTAL @colin-mccarthy PTAL goreleaser @caarlos0 PTAL evilsocket @caquino PTAL