HTML writer: Add warnings on duplicate attribute values.

This prevents emitting invalid HTML.

Ultimately it would be good to prevent this in the types
themselves, but this is better for now.

T.P.Logging: Add DuplicateAttribute constructor to LogMessage.
[API change]

test/command/duplicate_attributes.md

@@ -0,0 +1,7 @@
+```
+% pandoc
+[span]{.foobar style="color:blue" class="zip" style="color:red"}
+^D
+[WARNING] Ignoring duplicate attribute style="color:red".
+<p><span class="foobar zip" style="color:blue">span</span></p>
+```