[FEATURE REQUEST] typed arguments

If I write a typed variable argument, I am expecting an argument of a specific type.
Example:

```python
import fire

def test(some_str: str):
print(some_str)

if __name__ == "__main__":
fire.Fire()
```

Real result:

```shell
python test.py test 7.30
7.3
```

Expected Result:

```shell
python test.py test 7.30
7.30
```

So this feature will not break [argument parsing](https://github.com/google/python-fire/blob/master/docs/guide.md#argument-parsing). The typed argument should be enough to detect the real type.