This reverts commits 24cb8b1..d0abb37 from clap-rs/clap#1840
This is part of #16. clap-rs/clap#1840 wasn't the right call but we
don't have time to make the decision now, so instead of having one
option and changing it in 4.0, this reverts back to clap2 behavior.
This better models what users should be doing and makes it so all
comments are more clear. In a prior commit, when a changed an `exit` to
`unwrap`, I disliked the fact that I was mixing an unwrap explanatory
comment in with another comment. This makes them stand apart.
