Multi-Release JAR support

[Ladicek]

This is a mirror of JANDEX-43 from the Red Hat JIRA. That issue says:

In order to support MR JARs, Jandex needs some sort of way to produce different indexes for different releases (maybe separate files but not necessarily), and then to select and use the correct index based on the currently running Java version.

I'm not entirely sure how that MR JAR support in Jandex should look like or what are the use cases. Filing this just for tracking, but the issue in Red Hat JIRA is more than 4 years old and noone complained loud enough, so I don't think this has a high priority. Comments welcome.