Provide IPFS hashes in TimeMaps

[machawk1]

ipwb currently uses the hash "locator" (see #41) field in CDXJ's JSON block to retain the IPFS hashes for the HTTP headers and HTTP payload as extracted from a WARC and maintains the association with the URI-R and datetime.

ipwb currently is working toward full Memento compatibility (#105) but it may be useful to identify these same hashes in a TimeMap.

This would be not an issue with CDXJ TimeMap, as we could use a similar mechanism of storing the data in the CDXJ block. However, Link format syntax are more strict.

https://github.com/hvdsomp/signposting is working on introducing the "identifier" relation type and ties in persistent identifiers like DOI-URLs. IPFS hashes are extremely descriptive of the content is identifies.

Figure out a way to elegantly display this information in TimeMaps, both of Link and CDXJ format, for the potential interoperability with other tools.

[machawk1]

As of 5ecd56b, ipwb only supports Link formatted TimeMaps. #180 would add support for CDXJ and JSON TimeMaps.

In this ticket we ought to flesh out how a sample TimeMap with the hash should look. For example,
$ ipwb index ipwb/samples/warcs/5mementos.warc | ipwb replay then curl http://localhost:5000/timemap/link/memento.us returns

<memento.us/>; rel="original",
<http://localhost:5000/timemap/link/memento.us>; rel="self"; type="application/link-format",
<http://localhost:5000/20130202100000/memento.us/>; rel="first memento"; datetime="Sat, 02 Feb 2013 10:00:00 GMT",
<http://localhost:5000/20140114100000/memento.us/>; rel="memento"; datetime="Tue, 14 Jan 2014 10:00:00 GMT",
<http://localhost:5000/20140115101500/memento.us/>; rel="memento"; datetime="Wed, 15 Jan 2014 10:15:00 GMT",
<http://localhost:5000/20161231110000/memento.us/>; rel="memento"; datetime="Sat, 31 Dec 2016 11:00:00 GMT",
<http://localhost:5000/20161231110001/memento.us/>; rel="last memento"; datetime="Sat, 31 Dec 2016 11:00:01 GMT"