Include multiple reports during site-generation by defining additional goals

This is the second take of #105 as discussed in https://groups.google.com/d/msg/jacoco/I6Z43tfzoQc/DuSb_oZNYxYJ:

* To differentiate between the coverage of unit tests and integration tests, define additional goals `prepare-agent-integration` and `report-integration`. It is crucial to use maven-site-plugin version 3.3+
* See jacoco-maven-plugin.test/it/it-site-failsafe/pom.xml for an example how to use this.