Support for environment variables as default

As I user I'd like to be able to specify some of my arguments as environment variables.

Quite often I have a single bash script performing many calls to a script written using python-fire. Many of the functions defined in the python script contain shared arguments which I am just effectively passing from my environment variables to each of the calls. Having a way to pass those through would be extremely useful.

## Desired behaviour:

Given:
```python3
$ cat run.py
import fire
def function_1(arg1, arg2=1):
print(arg1, arg2)
fire.Fire(function_1)
```

I'd like:
```bash
$ arg1=arg1 python3.6 run.py -- --use_environment
arg1 1
$ arg1=arg1env arg2=arg2env python3.6 run.py arg1 arg2 -- --use_environment
arg1 arg2
$ arg1=arg1env arg2=arg2env python3.6 run.py arg1 -- --use_environment
arg1 arg2env
$ arg1=arg1env arg2=arg2env python3.6 run.py -- --use_environment
arg1env arg2env
$ arg1=arg1env python3.6 run.py -- --use_environment
arg1env 1
```

This would be extremely useful as then I'd be able to have the following:

```bash
export HOST=mydesiredhost
./machine_manager ping -- --use_environment
./machine_manager shutdown -- --use_environment
# No need to pass --host=mydesiredhost on each command
```