Add EventSourceResponse.last_event_id

[decaz]

I think it would be useful to add last_event_id property to the EventSourceResponse class from the example: https://github.com/aio-libs/aiohttp-sse/blob/aca7808960635ff7511ff46002b7f83168f6952c/examples/graceful_shutdown.py#L14-L16

Also there is a bug at that example (sse_response doesn't accept response_cls parameter): https://github.com/aio-libs/aiohttp-sse/blob/aca7808960635ff7511ff46002b7f83168f6952c/examples/graceful_shutdown.py#L63

[jettify]

Would you like to create PR with a fix?

[tommmlij]

The fix is already in master. Somebody mighty needs to release 2.2.0. Until then as a "workaround" I directly install from github instead of PyPI