Where is the `proxies` parameter of ClientSession? And how could I add proxy for whole session?

## Long story short

Document:

http://aiohttp.readthedocs.io/en/stable/client_reference.html#aiohttp.ClientSession

```python
class aiohttp.ClientSession(*, connector=None, loop=None, ..., proxies=None)
```

Real world:

https://github.com/aio-libs/aiohttp/blob/master/aiohttp/client.py#L53-L62

Well, what happened?

## Expected behaviour

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## Actual behaviour

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## Steps to reproduce

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## Your environment

aiohttp 2.3.5