[WIP] Add merge/rebase feature

[alexellis]

Description

Experimental command allowing merge (read rebase) via Derek command

Motivation and Context

• [ ] I have raised an issue to propose this change (required)

This would require additional ACL / RBAC

Raised because of backlog item: https://github.com/alexellis/derek/issues/91

How Has This Been Tested?

Tested with an older version of master with @viveksyngh e2e with a test repo.

We have two test repos:

mergers:
- people-who-can-say-derek-merge

must_approve:
- name-that-must-approve-the-pr-to-merge

https://github.com/alexellis/kumbaya/pulls

https://github.com/alexellis/two-better-than-one/pulls

Types of changes

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

Closes: #112