.names parameter for pack/unpack

[vorpalvorpal]

Currently pack/unpack have a .name_sep argument that packs/unpacks names as <Outer><sep><Inner>. I am working with a data set where the names are <Inner><sep><Outer>. There is no easy way of dealing with this set up. The .names parameter used by across deals with this easily by having a glue specification with {.col} being used as a mask. It seems like something similar would be simple to achieve for pack/unpack where .names would have masks for {.inner} and {outer}, perhaps with a default of "{.outer}.{.inner}".

If this is something you would consider, I can try to put together a PR.