html-proofer needs retry

[vsoch]

There is currently no variable you can set in html-proofer to allow for a retry, and this is a huge pain point because travis is failing every other time with a URL that otherwise is fine. There are two issues to track here - the first is for the underlying library:

• Add support for automatic retry, exponential back-off, and queuing for later. typhoeus/typhoeus#182

And the second then will need to bubble up and be represented here:

• Do twice check on HTTP error gjtorikian/html-proofer#310

I've been re-running them every time, but it's a PITA.

[vsoch]

Look into https://github.com/SuperKogito/URLs-checker

[vsoch]

urlstechie/urlchecker-action#17

[vsoch]

Done done done :)