Support Neel Nanda's counterfact dataset

[norabelrose]

In general, we should support datasets where the statements don't take the form of question-answer pairs. The first obvious dataset to support is counterfact.

[norabelrose]

Don't we just support this with @azhx's dataset now

[azhx]

I should clean up my branch, it's not committed yet. Been a bit busy lately sorry. Let me know when you would prefer this to happen

[lauritowal]

@derpyplops