Add error message when --dist is not specified.

[domsleee]

Fixes #354 by adding a descriptive message.

Previous behaviour

❯ npx gh-pages .
The "paths[1]" argument must be of type string. Received undefined

New behaviour

❯ npx gh-pages .
No base directory specified. The `--dist` option must be specified.

Other notes

I also used Command() instead of the singleton CommanderStatic, since it makes it more testable.
With const program = require('commander');, values like program.user were being carried from previous tests 👍

[tschaub]

Thanks, @domsleee