查看maven plugin有哪些goals

参考：http://www.avajava.com/tutorials/lessons/how-do-i-list-the-goals-of-a-maven-plugin.html

命令：mvn -sD:/.m2/settings.xml help:describe -Dplugin=war

等价于：mvn -sD:/.m2/settings.xml help:describe -Dplugin=war -Dmedium