CLIUtils/CLI11: Version 2.1: Names and callbacks

The name restrictions for options and subcommands are now much looser, allowing a wider variety of characters than before, even spaces can be used (use quotes to include a space in most shells). The default configuration parser was improved, allowing your configuration to sit in a larger file. And option callbacks have a few new settings, allowing them to be run even if the option is not passed, or every time the option is parsed.

• Option/subcommand name restrictions have been relaxed. Most characters are now allowed. #627
• The config parser can accept streams, specify a specific section, and inline comment characters are supported #630
• force_callback & trigger_on_parse added, allowing a callback to always run on parse even if not present or every time the option is parsed #631
• Bugfix(cmake): Only add CONFIGURE_DEPENDS if CLI11 is the main project #633
• Bugfix(cmake): Ensure the cmake/pkg-config files install to a arch independent path #635
• Bugfix: The single header file generation was missing the include guard. #620