Detect CID in subdomains

[lidel]

Parent issues: ipfs/in-web-browsers#89, #527

It is possible to use cidv1b32 as a subdomain to take advantage of origin-based security perimeter in web browsers. It should be fairly easy to detect if subdomain is a valid CID, and if so, display the same UI elements as for /ipfs/<cid> paths.

In these cases, the CID will always be in a zone named ipfs (or ipns), e.g. http://bafkreigh2akiscaildcqabsyg3dfr6chu3fgpregiymsck7e7aqa4s52zy.ipfs.dweb.link, so a regexp could look something like ^b[^.]{42,}\.ipfs\. (42 being a made-up minimum length for a base32 CID).