Add support for the '--' operator to mark the end of options (and so be able to handle arguments starting with '-')
Accept '-' as an argument value (useful for commands that read or write from stdin or to stdout)
Add support for -o=(true|false) for boolean options
Fix bug in option sequences handling and add a test
Help Messages Improvements

* Display env var name at the end of the description of an arg or opt (Closes #2)
* Add HideValue param to arg and opt to prevent displaying the value in the help (Closes #1)