feature request: allow passing unparsed parameters

a common idiom in cli is to have a flag (usually --, but this is taken here) that all text afterwards is passed as is (mostly for passing to other commands)

it could be useful to have a similar feature in fire

for example:

> example.py foo --- this is passed as is - --

example.py:
```
import fire

def hello(name, __unparsed__):
print(name)
print(__unparsed__)

if __name__ == '__main__':
fire.Fire(hello)

```
will yield:
> foo
> this is passed as is - --

從 Python Fire 的命令列解析入口開始，將要求的 `---` 範例與目前的引數處理進行比較。確定應如何表示未解析的文字，並透過一個有針對性的 CLI 測試驗證行為；當範例將剩餘文字原樣傳遞給 `__unparsed__` 且不破壞現有解析時，即表示完成。

由索引模型根據 Issue 內容生成。