Retries for potentially transient errors

[charleskorn]

First of all, thank you for this great tool, it's saved me a heap of time.

Occasionally, linkcheck will fail because an external site returned a HTTP 503 or something like that. It would be great if linkcheck could be configured to retry in this case rather than failing the entire invocation because one site is down.

[filiph]

For what it's worth, I really like this idea. I don't think I'll have the time to implement it myself anytime soon, though..