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'
Shows help if the default command was called without any arguments but has required arguments.
This way it does not hinder execution of a default command without any required arguments.
When called without args:
- Default command with required params -> Show help
- Default command with required params and additional commands -> Show help including additional commands
- Default command without required params -> Execute command
* Support negative numbers as command option values
* Support command line options before arguments
* POSIX-compliant handling of quotes (double and single, terminated and unterminated), whitespace, hyphens, and special characters (e.g. emojis)
