* Text.Pandoc.Writers.HTML: removed writeHtml, writeHtmlString,
added writeHtml4, writeHtml4String, writeHtml5, writeHtml5String.
* Removed writerHtml5 from WriterOptions.
* Renamed default.html template to default.html4.
* "html" now aliases to "html5"; to get the old HTML4 behavior,
you must now specify "-t html4".