支持启动时增强/trace/watch类

目前arthas的工作方式是主动的，当执行命令时，从jvm搜索所有加载的类，做一次增强。

* 如果命令执行时，类还没被加载，则搜索不到，也就是不起作用
* 如果命令执行成功后，又加载新的类，也会匹配不到

考虑一种Listener的模式，可以预先设定匹配的表达式，如果有匹配的类被加载时，就会触发增强。

难点在于怎么和现有的机制结合起来。

The issue names no files, tests, or entry points. Review the existing command-time class search and enhancement flow first, then determine how a listener could retain matching expressions and react when classes are loaded. Done would require an agreed design that integrates this behavior with the current mechanism.

Written by the indexing model from the issue text.