Implement demo of idiomatic crawler using aiohttp client

Lots of people using aiohttp client to crawl internet [1], I think to encourage good practices and idiomatic approach it is good idea to have specific demo for this purposes. Good starting point is [2]

[1] https://medium.com/@cgarciae/making-an-infinite-number-of-requests-with-python-aiohttp-pypeln-3a552b97dc95
[2] http://www.aosabook.org/en/500L/a-web-crawler-with-asyncio-coroutines.html