#尝试实现简单的插件功能

$ python main.py -h
usage: main.py [-h] [--enable PLUGINS]

optional arguments:
-h, --help        show this help message and exit
--enable PLUGINS  enable some plugins (default: [])

默认输出：

$ python main.py
Default: test plugin.

启用插件 foo:

$ python main.py --enable foo
Default: test plugin.
Plugin foo: test plugin.

启用插件 foo, bar:

$ python main.py --enable foo --enable bar
Default: test plugin.
Plugin bar: test plugin.
Plugin foo: test plugin.