title: Validate format of GitHub "Squash and Merge" commit messages
body: <p>I want to set up a GitHub project to use <a href="https://github.com/conventional-changelog/standard-version" rel="noreferrer">conventional-changelog standard version</a> to generate its changelog. We use a <a href="https://help.github.com/articles/about-pull-request-merges/#squash-and-merge-your-pull-request-commits" rel="noreferrer">squash and merge</a> workflow to merge PRs into our main development branch.</p>
  
  <p>Is there a way that I can enforce that the commit message of the squashed commit follows a specific format?</p>
  
  <p>Ideally this would work nicely with the GitHub UI, such as showing as a failing check. I realize I could simply write a browser extension to enforce this, but then everyone working on the project would need to install the extension which is too high of a barrier to entry for an open source project.</p>