addon-add and addon-remove commands

Here's how this looks for addon-add:

```bash
$ hk addon-add heroku-postgresql

$ hk addon-add heroku-postgresql:ronin fork="postgres://U:P@ec2-12-123-123-123.compute-1.amazonaws.com:5752/dabcdefghjiklm"
```

So it's currently got a `key=value` format for config options. I'm not honestly sure why I implemented it that way, I guess I just wasn't thinking about the `--key value` or `--key=value` format of the toolbelt's `addons:add` command.

Obviously, I don't have any of the [postgres config var translation stuff](https://github.com/heroku/heroku/blob/701c612b236ca0b886f78d73384593a749a0566d/lib/heroku/command/addons.rb#L270-L280) right now so you have to specify the actual URL as a value for things like fork/follow. Wouldn't be surprised if most people weren't aware of how that worked under the hood. I wonder if @pedro and @brandur have ideas on how to provide a proper way of delegating access for this sort of thing.

Still working on addon-remove.