Main changes:
- Use the `columnify` module to display the commands instead of using
manual parsing.
- Extract logic to create a string representation from an option
signature to Capitano, and reuse here.
See https://github.com/resin-io/capitano/pull/28
Some bugs were caught and fixes during the refactoring:
- In command help, if the command didn't exist, we reused default
Capitanos command not found function which uses `process.exit(1)`. This
was changed to pass a custom error to `done()`, so the command fails
correctly when using programatically.
- General help didn't call `done()` at all, thus causing problems if
using the command programatically someday.
