Allow boolean flag with .demand()

This change allows options to be demanded with `.usage()` and `.options()`, without having to give a message argument, for example:

    yargs.options({
      'input': { description: 'Input filename', demand: true }
    })

I'm wondering whether to make `required` a synonym for `demand` in this case (see https://github.com/chevex/yargs/issues/28). Please let me know and I'll update this pull request with the change.

Cheers,

Chris