Add the ability to show several sets of changes

This request builds on CHANGELOG-61, which adds the ability to create a log based on an absolute date or tag. We would like to be able to extend that thinking so I can create a report that shows changes for several time intervals, based on dates or tags.

For example, I would set maven.changelog.tag=TAG_1,TAG_2,TAG_3 (and maven.changelog.type=tag), and the report would show all the changes between TAG_1 and TAG_2, followed by all the changes between TAG_2 and TAG_3, followed by all the changes between TAG_3 and the present time. Similarly, I could do the same thing with a list absolute dates.