Add prop

I guess to add `aiohttp.web.StreamResponse.started` property.

It should be `False` if response has not started and `True` when `response.start()` has been called.

The reason is: in custom middleware I want to replace response object for, say, rendering pretty looking page for 500 status code.

But if response has been started (it's response for processing streaming video for example) I cannot do that.

Even catching `RuntimeError` from `.start()` call cannot solve it if I want to **replace** the response, not modify it properties.

Objections?

If not I'll prepare full PR with tests etc.