Expose the generated properties not only in project.properties

[TheSnoozer]

Describe your idea

Some plugins seem to get the properties from System.properties or other places (e.g. see sonar-plugin or the flatten-maven-plugin)

Expected Behavior

By default this plugin should also expose it's generated variables to all sort of locations.

Potentially have a switch to turn that off.

[TheSnoozer]

Fixed with #414

There is now also a new switch injectIntoSysProperties which would disable the population into System.getProperties(). Arguments from command-line seem to have precedence.