add failing test and fix for #91

[aaronpk]

only looks on a,link, and area for rel attributes, since these are the only elements that HTML allows to have the rel attribute.

[kevinmarks]

From IRC:
@aaronpk BOT [2:39 PM]
I only have tiny internet right now so can't comment on my PR, so can someone add this as a comment? Don't merge my php-mf2 PR yet because I just realized it's wrong and I'm going to add another test case to webmention.rocks tomorrow, and I have a plan to fix the PR

[aaronpk]

It appears XPath does not have the problem @tantek and I suspected after all. We were assuming that since the query used an "or" expression at the top level, which XPath treats as a "union", that it would return the tags in the order they were in the query. However I've added tests and it looks like XPath is still returning the tags in document order. This is fine to merge now!

[barnabywalters]

Had brief discussion at IWC Nürnberg with @tantek @aaronpk about whether to future proofing or HTML spec compliance is higher priority, decided that HTML spec compliance takes priority. Thanks for your work on this @aaronpk!