Add subSelect() helper function

[pavarnos]

See #37 (comment)

[shadowhand]

How would this operate any differently than $select->from(alias($inner, 'x')) ?

[pavarnos]

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?

[shadowhand]

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().

[pavarnos]

ok. want me to delete this PR?

[shadowhand]

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