POSIX compat

[felixSchl]

Neodoc is already mostly POSIX compatible, but there are a few things that can be improved:

• "Options-first" on by default (guideline 9)
• Required option arguments be separate from the option: -f BAR
  • Update notation in usage syntax (disallow no spaces)
• Optional option arguments be the same argument as the option: -fBAR
  • Update notation in usage syntax (disallow spaces)
• Denote options in singleton groups: [-f BAR] means [-f=BAR] (no
  option description required)
• Allow passing negative numbers as option arguments #39
• Implement guideline 8 (also see 11)