Add between

[ibarrae]

Fixes #34

Please, let me know if there's anything else to be done/fixed regarding this issue 👍

[parsonsmatt]

Looks great! Thanks 😄

[parsonsmatt]

Unfortunately this implementation doesn't work with non-val values. Here's a failing test case:

    it "works for a SqlExpr value" $ run $ do
        p1e  <- insert' p1
        _    <- insert' p2
        _    <- insert' p3
        ret <-
          select $
          from $ \p -> do
          where_ $
            just (p ^. PersonFavNum)
              `between`
                (p ^. PersonAge, p ^.  PersonWeight)
        liftIO $ ret `shouldBe` []

This fails with a format error. It should eventually succeed and then the [] return value should be filled in appropriately.

I can't figure out how to get this PR unmerged, so you will have to open a new PR :\