Bringing back support for declaring proxies at a init level in ClientSession().

## Long story short

Currently only http proxies are supported and have to be declared at a request level.
I'd like to see support for declaring proxies at the initialization level of `ClientSession()`.

Third party libraries that add SOCKS support already have to do this because of needing a `connector`.
```python
self.c = SocksConnector.from_url('socks4://132.148.129.183:47617')
self.s = aiohttp.ClientSession(connector=self.c)
```

This makes providing support for both HTTP proxies _and_ SOCKS proxies really awkward.

## Expected behaviour

I'd like to see native HTTP proxy deceleration in the same manner as is provided for SOCKS proxies by third party libraries.

## Actual behaviour

Currently we have to declare HTTP proxies at a request level.
```python
def request(self, method, url, **kwargs):
return self.s.request(method, url, **kwargs, proxy='http://192.168.178.45:8080')
```