HTML writer: insert data- in front of unsupported attributes.

Thus, a span with attribute 'foo' gets written to HTML5
with 'data-foo', so it is valid HTML5.

HTML4 is not affected.

This will allow us to use custom attributes in pandoc without
producing invalid HTML.

test/command/custom-attributes.html

@@ -0,0 +1,16 @@
+Custom attributes should automatically have data- added
+in HTML5:
+```
+% pandoc -t html5
+[hello]{foo="bar"}
+^D
+<span data-foo="bar">hello</span>
+```
+
+but not in HTML4:
+```
+% pandoc -t html4
+[hello]{foo="bar"}
+^D
+<span foo="bar">hello</span>
+```