Proposal: Label and message for PRs with Merge Commits

If there's a merge commit pushed to a PR can we add a label and post a message to the PR from Derek saying that Merge Commits are not allowed, and provide info on how to remove it, and rebase.

Can we detect easily merge commits? Is it the format of the commit message or are they different?

## Expected Behaviour

A PR with a merge commit pushed to it should get a label from derek and a message posted about why and how to fix it.
2 scenarios
1) A PR is raised and there is already a merge commit
2) A Merge Commit is pushed to the PR

Should this be optional? Configured in the yaml? What about for repos that dont have the config set? Do we enable or disable by default?

## Current Behaviour

No feature
## Possible Solution

Add a check into Derek that fulfils the 2x scenarios if enabled.