Change nm_join() and nm_data() to optionally not return integer

[kylebaron]

Needs discussion; but I think it might simplify things downstream. To clarify, R & data.table do it slightly different than readr; if we did this change, we'd just be moving toward making the output more like what readr returns.

Also, I'm going to do everything I can with the downstream stuff to handle this well; I think I can do that and don't want to make this just about our downstream tooling.

[seth127]

Thanks Kyle. Would making the change suggested in #722 accomplish this?

[kylebaron]

@seth127 No; I think #722 only addresses long integers, not integer to numeric / double. Related but not the same so I made two issues.

[kylebaron]

@seth127 one way this is related: if we ask data.table to return long integers as doubles, then it would make sense to return all ints as doubles out of consistency.