lists for addon services and plans

I'll need these in order to be able to offer completion of these values.

Examples:

    $ hk addon-services
    heroku-postgresql
    newrelic
    redisgreen
    ...
    
    $ hk addon-plans heroku-postgresql
    hobby-dev        $0/mo
    hobby-basic      $9/mo
    standard-yanari  $50/mo
    standard-tengu   $200/mo
    premium-yanari   $200/mo
    premium-tengu    $350/mo
    standard-ika     $750/mo
    premium-ika      $1200/mo
    ...

/cc @max for design review