if __name__ == '__main__' for main examples.

https://github.com/aio-libs/aiohttp-devtools/issues/229

Would be useful if we wrapped `web.run_app(app)` if a main check:

```py
if __name__ == '__main__':
    web.run_app(app)
```

To work with aiohttp-devtools and other such tools.

If you're happy with this I'll submit a PR.
