add example to all commands

[jdx]

we should go through every command and ensure that they have examples that display output

• TODO: list all commands here to be checked
• (maybe) add .examples[] property to more easily list in dev center

proposed example format:

static examples = [
  {
    description: 'Upgrade an add-on by service name',
    command: 'heroku addons:upgrade heroku-redis:premium-2',
    output: '...'
  }
]