Add subSelect() helper function

See https://github.com/shadowhand/latitude/issues/37#issuecomment-651809904

no difference But it makes it explicit and it creates a clean obvious syntax. I spent a couple of hours trying to figure out how to do a subselect: lots of googling until I stumbled across #37 and its related comment: it is not mentioned in your docs anywhere (that i could find) but is a very common thing to want to do. Maybe think of it as a DX improvement?

I think it would be better to add an example to the documentation than add another function that does exactly the same thing as alias().

ok. want me to delete this PR?

Or you can replace this PR with one that updates the docs. Your choice.