Change default help template:
- The new template introduce new lines before and after
author/about sections.
- Add help template placeholders:
- about-section
- author-section
- Documentation of new placeholders in clap::App::help_template
- Update all unit tests by incorporating new lines
Tests to check for conflicts between flag subcommands long and short and
their aliases and args both long and short and their aliases. Tests to
handle self conflicts, where a flag subcommand short or long with have a
corresponding alias with the same value.
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`.
