Allow for `body-template` updates

[HartiganHM]

Hey, all!

It would be great to be able to replace a portion of the body description via regex instead of a complete replacement. Right now, I'm working on updating each PR dynamically with a release version which will change each time the PR is updated. This would allow me to find the desired text in the body and update it.

// PR is opened
- Release version: [1.0.3](<link-to-registry>)

// PR is synchronized
- Release version: [1.0.4](<link-to-registry>)

Happy to update this as necessary with more information. Thanks for the awesome GH Action!

[tzkhan]

Hi @HartiganHM, thanks for the feature suggestion! Being able to replace portions of the PR body sounds pretty handy.

Just so I could better understand the scenario, could you provide a sample config with the intended behaviour you're after.