Support `value` customization (aggregate values!)

To quote @skosch:

That’s really neat (and should be documented!), but I still think there’s value in your earlier proposal, since often enough the cells combine data from multiple nested fields, i.e. ${dad.firstName} ${dad.lastName}, and we’d like to have that searchable too, no?

[bebraw]

The search case isn’t actually trivial. Search.search needs to output unflattened data so the surrounding logic doesn’t break. So it needs to do some sort of lookup while keeping the original shape of the structure intact. I realized this while playing around with the ideas earlier. But that’s just details. 👍

[bebraw]

I ended up implementing value field after all and it works quite beautifully. So I guess it’s safe to close this then. The feature will make it to the next beta (due later today).