Allow explicit configured plugins to execute on master as well

[glimmerveen]

Extended the MasterPromoteExtension to not only retain the hardcoded gitflow-helper-maven-plugin itself and the maven-deploy-plugin, or the plugins related to explicit supplied command-line goals, but also allow a project to configure additional plugins that may also execute on master/support.

This serves two cases:

1. It solves the issue #127 (though admittedly it still would be more clean to be able to download the pom file from the release repo)
2. It can serve as a basic extension point to allow other "things" to happen on a master build, that are beyond the scope of deploying maven artifacts (ie have some antrun script run)