[whatwg] WA1: rev attribute

[fantasai]

The 'rev' attribute from prior versions of HTML is missing in WA1,
and I think it deserves not to be left out. Most common link types
out there are used with 'rel', but some 'rev' values can also be
useful. Here are some use cases:
   - rev="footnote" for a link back from the footnote or endnote to
     the source anchor in the main text
   - rev="help" for a link to the part of the site that the help
     text is about
   - rev="author" on a personal site or resume for links to documents
     s/he has written

See also http://www.eastgate.com/HypertextNow/archives/Trigg.html
for a direction link types could go in which 'rev' would be useful.
Many of the link types suggested there would be easier to use with
rev for the reverse link than with a separate keyword that means
the inverse relationship.
Example:
   rev="refutation" to link to the article one is refuting

~fantasai