Faraday's retry configuration is not exposed

Faraday includes [a retry middleware module](https://github.com/lostisland/faraday/blob/master/lib/faraday/request/retry.rb).

At present, `droplet_kit` does not expose this thus retries are not configurable _unless_ the client's `initialize` and `connection` methods are overridden into a new client (as far as I can tell).

An enhancement here would produce an interface something like this

```ruby
client = DropletKit::Client.new(
access_token: 'YOUR_TOKEN',
retry_exceptions: [CustomException, 'Timeout::Error'],
retry_statuses: [500, 501]
)

# or I think I prefer this:

client = DropletKit::Client.new(
access_token: 'YOUR_TOKEN',
retry: {
exceptions: [CustomException, 'Timeout::Error'],
statuses: [500, 501]
}
)
```