Add support for retrying failed requests.

[andrewsomething]

This enables support for retrying failed requests. A few things to note:

• I've chosen to expose the http-retry-max flag to users to give them insight into the new default behavior. I've marked the http-retry-wait-max and http-retry-wait-min flags as hidden as the defaults should be fine for most users, but it still allows for configuration if needed.
• For the wait settings, I've used an int rather than a float64 as I feel that it is friendlier on the command line.
• I've used the http prefix on these so that the environment variables end up consistent with the same settings ion Terraform, e.g. DIGITALOCEAN_HTTP_RETRY_MAX.
• I've bypassed retries for the doctl account ratelimit command. The purpose of the command is to return the rate-limit headers regardless of response status. So retrying here just hides the current status and delays returning a response for no real gain.

[bentranter]

LGTM!