support `--` for negative number and other input with dash

[JesseEisen]

When use -- to supports negative number, it only support one digit like -5, and not support -500. it Will treat -500 as short flag. So i just make the argument after -- as an entirety. now it support flag like this:

$ ./test -r a -ov  -- -help --halo
required: a
optional: (null)
verbose: enabled
additional args:
  - '-help'
  - '--halo'

It will not treat -help as -h,-e, -l,-p.