New parameter for additionalMergeOptions

[Iridias]

The available parameters to control the merge-behavior are quite limited.
Recently we came across the need to specify additional options, in order to make the release-process more robust.

I introduced a new parameter additionalMergeOptions in order to be able to specify options like -Xtheirs to avoid nasty merge-conflicts.
E.g. when specifying "-Xtheirs" for "gitflow:release" it will take the changes from staging, when encountering a merge-conflict. And as the versions for the release are updated in the staging-branch beforehand, it is ensured, that you don't end up with snapshot-versions in production-branch.

[Iridias]

As we need even more flexibility, I introduced more parameters for hotfix-finish and release-finish in order to have different merge-options for production-branch and development-branch.