feature: add support for archive.is

in addition to the wayback machine, archive.is is another good archiving site – one big advantage with archive.is is that it rightly does not follow robots.txt, thus content can always be archived and and remain in their DB even if the domain changes hands and the new owner adds a robots.txt to prevent archiving

this is a problem with the wayback machine – sometimes a domain will be taken over by someone who does not want the old content to be archived, so they add a rule in robots.txt and *poof* it’s gone from the wayback

https://wordpress.org/plugins/broken-link-checker/