Ability to extract sections from the PR for use in commit template

[jaycle]

Great that we now have #612 for more control over the commit messages.

Mergify has a feature to extract components from the PR description using a syntax like {{ body | get_section("## Description") }}

see: https://docs.mergify.com/configuration/#data-type-template

With something like this, one can designate a section that is slated to become part of version control history. I would love to combine something like that along with forced squash to allow for reviewers to actually review the content of the commit before approval. Having the entire PR body is a good step, but for many workflows, it contains a lot of detail and examples that are more relevant to the review than a statement of the change.