Improved printing instead of printing out the `Vec` using `{:?}`,
`join(", ")` for improved printing. Improved documentation to b emore
explicit and apply to removal of `FlagSubCommand` -> `App::short_flag`
and `App::long_flag`
Instead of a `FlagSubCommand` struct the addition of two simple methods
to `App`. `App::long_flag` and `App::short_flag` that cover all the
bases of the many methods that were provided in `FlagSubCommand`. This
API is far simpler to use and more akin to the present `Arg::long` and `Arg::short`.
