This ports our example testing over to [trycmd](https://docs.rs/) so
we can:
- More thoroughly test our examples
- Provide always-up-to-date example usage
The old way of testing automatically picked up examples. This new way
requires we have a `.md` file that uses the example in some way.
Notes:
- Moved overall example description to the `.md` file
- I added cross-linking between related examples
- `14_groups` had a redundant paragraph (twice talked about "one and
only one"
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.
