[Clopts] Proper GNU short option parsing

[Sirraide]

We already sort of support short options, but the way they work is pretty limited atm. Finishing implementing short options entails:

• Parsing short option clusters (e.g. -la in ls -la).
• Parsing short option clusters that contain exactly one option that accepts arguments (e.g. -xvf in tar -xvf .... This option must come last (e.g. tar -fvx is not valid), and as a consequence, it need not be separated from its argument by whitespace.
• Should probably also add an option that lets the user specify whether -- should stop parsing.
• Actually documenting all of this.
• multiple<short_option<...>> does not seem to work at the moment.