Support dry-run mode

[adangel]

Especially for testing the script of a release build, a "dry-run-mode" would be useful.

For releases, the publishing of the release could be skipped - so that the draft release can still be verified.
For maven central, the automatic release of the staging repo can be skipped, then we are left with a staging repo, that can be used for testing and dropped afterwards.
File uploads to sourceforge/pmd-code.org need to be skipped (binaries, regression tester, docs).
Blog post on sourceforge can be created as draft, but the publish can be skipped.