default values should be visible in help screen

As per #114 the default value for an argument should be visible in the help screen.

Some considerations:
- If the default value is a simple type and not too long in length, we can just show it.
- If it doesn't serialize well (either because there's no good way to serialize it, or because its serialization is too long to show comfortably), then we need to determine a clean alternative to showing the default. Using the type may be one option for this alternative.

首先閱讀 #114 中的討論，並追蹤 Python Fire 如何產生 CLI 說明畫面以及取得引數預設值。定義什麼樣的預設值算是簡短且可序列化的，並定義對於過長或難以序列化的值所使用的 fallback；當說明輸出為每個引數顯示適當的預設值時，即視為完成。

由索引模型根據 Issue 內容生成。