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.
Add support for converting command parameters that doesn't have a built-in TypeConverter but has a constructor that takes a string. For CLI apps, FileInfo and DirectoryInfo will likely be the most useful ones, but there may be others.
When a conversion to an enum fails, list all the valid enum values in the error message.
Message before this commit:
> Error: heimday is not a valid value for DayOfWeek.
Message after this commit:
> Error: Failed to convert 'heimday' to DayOfWeek. Valid values are 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'
