Failing when the version is missing should be optional

The announcement goal fails if the current version is not added on the <versions> element. Although this is nice, sometimes it might not be convenient (for instance, you might have a project made of several sub-projects and you would want to run the announcement on each sub-project using multi-project).

So, we should add a new property, like maven.announcement.versions.ignore, whose default value is false (to keep backwards compatibility).