Support git-style "--" argument behaviour

Hi Giacomo,

I needed to use commandline in a plugin-like scenario, but where I only wanted arguments to be interpreted at the start of the command line, up to the first non-option argument. In my case, we have a command line utility which roughly speaking represents a flexibly-structured pipeline of files and some command tokens (# in the example). There may be some global options, and each command token might have its own options too, which could quite possibly have the same names as the global options. For example:

myapp -v5 file1 # -v3 file2

So I need to:
- Process any global options which are at the start of the command line
- Pass all following arguments to a token parser
- The token parser will also check for options after the command token (-v3)

My options class:

    class GlobalOptions 
    {
        [Option( 'v' )]
        public int Verbosity { get; set; }

        [ValueList( typeof( List<string> ) )]
        public IList<string> RemainingArgs { get; set; }
    }

By default, commandline will consider -v5 and -v3 as global options. By setting ParserSettings.DoNotInterpretAfterFirstNonOption to true, the parser stops trying to interpret options when it sees file1, and puts file1 and all subsequent arguments into RemainingArgs (including the -v3).

If the filename "file1" actually begins with a dash, it could be a problem, for example:

myapp -v5 -filename_starting_with_dash # -v3 file2

Many git commands have a workaround for this scenario: the double dash argument. The equivalent behaviour can be enabled by setting ParserSettings.EnableDashDash to true, and using:

myapp -v5 -- -filename_starting_with_dash # -v3 file2

I hope you'll see the usefulness of these changes and consider pulling them.

Many thanks,

Tom Glastonbury