exposing list_flatten

[bnicenboim]

Could you expose list_flatten? It would be nice to avoid a dependency to purrr.

[markfairbanks]

Feel free to just copy the code from my repo and use list_flatten() where you need it. I've decided I'm not going to export any more purrr functions from tidytable. There are just way too many for me to recreate all of them. In hindsight I probably shouldn't have exported map() functions either but they're going to stay at this point.