Support `select` function

[onomated]

Expand QueryBuilder to allow custom selection of fields returned reflecting Ecto's query select. Convo started on ElixirForum.

[mathieuprog]

Do you have any plans on working on this?

[onomated]

Haven't given much thought to it, but don't mind taking a stab at it. There's the select and select_merge functions in Ecto. I'm thinking we retain the same interface vs assume subsequent select calls in query builder auto merge vs replace