feature: retry support #88

Hi,

Does this module support retries?

To try to make a case for the importance of such a feature:

• they are just very useful in real-world scenarios when you hope to make downloads more reliable
• wget automatically retries
• curl optionally allows retries
• Invoke-WebRequest includes the options -MaximumRetryCount -RetryIntervalSec

The reason to mention Invoke_WebRequest is, those would be the sorts of options to add: "count" and "interval".

However, you may reply "There's this other library which already does exactly what you want. Switch to that. Simple-get won't add complex features, because it's 'simple'. " Not sure...

Thanks.