Support interlaced positional args and options

This commit adds the ability to parse interlaced arguments and options.
For example, the following:

    GBCli foo -o bar --print-settings

will now be parsed as:

    Arguments:
        foo

    Options:
        output         = bar
        print-settings

without needing to place all arguments after options.
Add special "end-of-option" tokens - and --
Need to add interlacing arguments to array
Fix group lookup error introduced in 7fdf6d5
Merge pull request #10 from uranusjr/positionals

Add support for interlacing arguments and explicit option ending