Implement offset

I've made this a separate pull request since it depends on installing a very recent haskelldb from the source repo, because it needs commit [cc7c53ac](https://github.com/m4dc4p/haskelldb/commit/cc7c53ac10727d2d905705bb78f9f1ed8ddf9750), and I wasn't sure if you want that. If you merge this branch, you'll also get limiting (#64).