[MJLINK-83] Implement multiple launcher elements

[pedro-w]

This is a simple change to allow the Maven plugin to generate more than one launcher in a jlink image. The jlink tool itself already supports this, but there was no way to pass the required invocation onto it from Maven.

Following this checklist to help us incorporate your contribution quickly and easily:

• [X] This JIRA issue has been filed for the change
• [X] Each commit in the pull request should have a meaningful subject line and body.
• [X] Format the pull request title like [MJLINK-XXX] - Fixes bug in ApproximateQuantiles, where you replace MJLINK-XXX with the appropriate JIRA issue. Best practice is to use the JIRA issue title in the pull request title and in the first line of the commit message.
• [X] Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• [X] Run mvn clean verify to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.
• [X] You have run the integration tests successfully (mvn -Prun-its clean verify).

If your pull request is about ~20 lines of code you don't need to sign an Individual Contributor License Agreement if you are unsure please ask on the developers list.

To make clear that you license your contribution under the Apache License Version 2.0, January 2004 you have to acknowledge this by using the following check-box.

• [X] I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

• [ ] In any other case, please file an Apache Individual Contributor License Agreement.