Fixes #10. Copy plugin configuration to all executions without a configuration.

I'm not sure why `plugin.configuration` is not passed to plugins (and `plugin.executions.execution.configuration` is). This might be a maven bug.
Here is some random method that uses `Plugin.getConfiguration` like `MavenProject.getGoalConfiguration`:

    dom = (Xpp3Dom) plugin.getConfiguration();
    
    if ( executionId != null )
    {
        PluginExecution execution = plugin.getExecutionsAsMap().get( executionId );
        if ( execution != null )
        {
            // NOTE: The PluginConfigurationExpander already merged the plugin-level config in
            dom = (Xpp3Dom) execution.getConfiguration();
        }
    }

It looks that putting `plugin.configuration` in `plugin.executions.execution.configuration` when it's `null`, is consistent with maven expectations.