Proper support for name attribute in <a> and <img>

Prior to this commit, the name attribute was unilaterally removed, except
for Strict doctypes or a heavy TidyLevel, when it was converted to an id
attribute. As name is actually permitted in both HTML 4.01 Strict and
XHTML 1.0 Strict, although deprecated, the more sensible default behavior
is to allow it unless TidyLevel is heavy.

Our implementation is slightly stricter than the specs, as name attributes are
treated as first class IDs, disallowing <a name="foo" id="foo"> or duplicate
names. The former should be treated as a special case, but that will be
a separate commit.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

TODO

@@ -14,7 +14,7 @@ afraid to cast your vote for the next feature to be implemented!
 - Investigate how early internal structures can be accessed; this would
   prevent structures from being parsed and serialized multiple times.
 - Built-in support for target="_blank" on all external links
-- Gitify the repository
+- Allow <a id="asdf" name="asdf'>
 
 FUTURE VERSIONS
 ---------------