用于在velocity中实现模板的继承,包含 block,override,extends指令
在velocity的properties中增加如下配置以激活自定义指定
userdirective=cn.org.rapid_framework.velocity.directive.BlockDirective,cn.org.rapid_framework.velocity.directive.OverrideDirective,cn.org.rapid_framework.velocity.directive.ExtendsDirective