Add support for CHANGELOG.MD as a changelog source for visualization

[oleg-nenashev]

There are many plugins which use CHANGELOG.md, e.g.

• https://github.com/jenkinsci/workflow-job-plugin/blob/master/CHANGELOG.md
• ... many other Pipeline plugins
• https://github.com/jenkinsci/compuware-topaz-utilities-plugin

It would be great to add support for this source if the information cannot be retrieved from GitHub Releases

[timja]

Worth looking at something like: https://github.com/hypermodules/changelog-parser I assume, although can't remember if changelog comes from here or plugin site api?