Redirect redd.it/xxxxxx shortlinks

[DokterKaj]

Closes #991. subdomain[1] will be undefined when there is nothing before redd.it in the string but since there's nothing else to test I just made it default. To be rigorous it can be changed to case undefined though

[DokterKaj]

Close #1004 as well

[ManeraKai]

You can use (?:) to not capture the match.

[DokterKaj]

I'm purposely capturing (and using) the match since there's no good way to detect both an optional subdomain without the dot and the presence of redd.it using only positive lookaheads
e.g. /^(?:(?:(?:external-)?preview|i)\.)?(?=\.redd\.it)/ keeps the dot and /^(?:(?:(?:external-)?preview|i)(?=\.))?(?=redd\.it)/ doesn't work