Add Regsiter, which allows go packages to define their own command line args

[alexflint]

Add Regsiter, which registers a struct containing command line arguments for inclusion in all future calls to arg.Parse or arg.MustParse. This allows you to define command line arguments per-package like this:

        package foo

        import "github.com/alexflint/go-arg"

        var args struct {
        	CacheSize int `arg:"--foo-cache-size"`
        }

        func init() {
        	args.Register(&argsFoo)
        }

Thanks jcarr!