Support UNLINK command

[zsblevins]

Swapping calls from delete to unlink yields:
redis.exceptions.ResponseError: Command # 1 (UNLINK <key>) of pipeline caused error: unknown command 'UNLINK'

[zsblevins]

Why it's worth implementing: https://redislabs.com/blog/the-little-known-feature-of-redis-4-0-that-will-speed-up-your-applications/

I can patch it in my tests as needed if there is no plan on implementing it

[bmerry]

I don't currently have any plans to implement it (because I have very little time to spend on fakeredis), but on the other hand I think it should be very easy to implement since it can just call del_. I'd be happy to review a pull request.

[zsblevins]

Here you go! #250