Infer expect

[ChristopherChudzicki]

This addresses #150 for ItemGraders only. Implementing this for ItemGrader only was very easy because ItemGraders have a strictly specified answers schema.

I suspect we could do something that works in general, but I would need to think a bit more about it.

I wrote this fairly quickly in case you wanted it for today's workshop. I think it should be safe to merge, but feel free to reject if you want a more general solution =).

[jolyonb]

This is exactly what I was after -- I'm a bit surprised it was this simple though! I found that I've been copying code a lot and just changing the "answers" tag, which was getting really annoying. Also, copy paste errors where I put the answer in the expect but forgot to change the answers tag... We should update documentation to reflect this too, but I'm happy with this for the moment.

[ChristopherChudzicki]

I found that I've been copying code a lot and just changing the "answers" tag, which was getting really annoying

Yeah, I can see that. For all the work I've done on this library, I've probably only written a dozen problems outside the demo course, so I forget about workflow issues sometimes 😦

We should update documentation to reflect this too

Yes!