release: support scmBuildNumber and scmTag attribute

I think it's very important to state the buildNumber and the tag of a release in the change log.

I suggest to add 2 optional attributes to the release element:

• scmBuildNumber
• scmTag

One could then set

• scmBuildNumber to ${buildNumber} (overriden by Build Number Maven Plugin; initialize to -1?)
• scmTag to ${tag} (overriden by Maven Release Plugin; initialize to ${project.build.finalName}?)

Of course this means that filtering should be supported (which would be a good thing anyway).