Access to command name

It would be helpful if I could access the command-name (the basename of `process.argv[1]`) from the opts structure, so I can use it in other error messages. Something like:
```
opt.parseSystem();
if (!parseFile(opt.argv[0])) {
  console.error(`${opt.command}: could not parse file ${opt.argv[0]}`);
  process.exit(1);
}
```
