Add unconsNonEmpty combinator to Pull

[Daenyth]

It's often useful to write pull with uncons, and skip over empty chunks by pulling the next chunk. This combinator makes sense to be in the library

example impl from @mpilquist: https://gitter.im/functional-streams-for-scala/fs2?at=5d162e628dff05627b84bebe

    def unconsNonEmpty: Pull[F, INothing, Option[(Chunk[O], Stream[F, O])]] =
      self.uncons.flatMap {
        case Some((hd, tl)) =>
          if (hd.isEmpty) tl.pull.unconsNonEmpty
          else Pull.pure(Some((hd, tl)))
        case None => Pull.pure(None)
      }

[monadplus]

I was wondering in which use cases you will be pulling empty chunks 🤔