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

先閱讀 cmd.js，找出判斷是否指定了特定 flag 的邏輯。將這段邏輯分離到獨立的檔案中，以便獨立測試，並在需要時使用模擬函式。完成的標準是：flag 偵測具有專注的測試，而且這些測試不會同時測試每個 flag 的作用。