I got some inspiration from Oleg Nenashev on Jenkins' mailing list, so I restructured the library and the plugin so that the Jira filter is just one possible output format of the changelog.
The key feature of the plugin is the automatic search of the latest tag and the changelog generation.
For now, there are only two output formats defined: a simple, human readable git history (similar to git log), and the mentioned Jira filter. See the README on github or the Wiki page for some basic examples to get started.
The code is available on github, pull requests are welcome!