Allow custom help providers
* Version option will show in help even with a default command
* Reserve `-v` and `--version` as special Spectre.Console command line arguments (nb. breaking change for Spectre.Console users who have a default command with a settings class that uses either of these switches).
* Help writer correctly determines if trailing commands exist and whether to display them as optional or mandatory in the usage statement.
* Ability to control the number of indirect commands to display in the help text when the command itself doesn't have any examples of its own. Defaults to 5 (for backward compatibility) but can be set to any integer or zero to disable completely.
* Significant increase in unit test coverage for the help writer.
* Minor grammatical improvements to website documentation.
