Refactor some cmd.js logic out into its own file for easier tests

While working on issue #69 I started to come across places where it was a little difficult to test if just a particular flag was called. I think it might help if some of the logic that is in cmd.js is broken out into its own file, so it would be easier to test if a particular flag was specified, and not necessarily test what that flag actually does. That could be done in its own separate test, where it would be easier to mock certain functions.

@richardlau @Trott thoughts?

I can send a PR so we can talk about it also

首先阅读 cmd.js，并找出用于判断是否指定了特定 flag 的逻辑。将这部分逻辑拆分到单独的文件中，以便独立测试，并在需要时使用模拟函数。完成的标准是：flag 检测具有专门的测试，且这些测试不会同时测试每个 flag 的作用。