Provide the ability to control commit messages

The commit messages used by maven-release-plugin are rather ugly to my eyes. While the comment prefix provides some control, what is really needed is to be able to change the whole message from xml configuration.

I've implemented this here:
https://github.com/jodastephen/maven-release/commit/35b2a50900cfc8e8fca88a94e46985a09e538e86 (all rights granted to Apache - I'm still a committer at Apache Commons.

It adds four new properties that allow the commit message to be controlled at various points. The commit message can be configured using `@{}` style inserts. There are more messages that could be controlled, but these were all I needed.

Note that issue MRELEASE-706 originally requested this feature, but it was closed due to inactivity.