os.environ['value'] support

That would be great to extend current syntaxis with standart python dictionary's.

For now we can work with .env vars like this: `os.getenv('value')` or `os.environ.get('value')`.
But `.get()` methods handles KeyError exceptions, so, we can't autocheck .env file correctness.

All we can do is to write smth like that:
```
foo = os.getenv('bar', None)
if foo is None:
raise KeyError('bar')
```
But, this of cource this ruins the code clearness/elegancy.

So, the main idea is to extend support for python dictionary syntaxis, which will looks like that: `os.environ['value']`.
This syntax will throw the exception in case of incorrect .env key.