Add HTML output

[stefanw]

HTML output works better than PDF output, as phantomjs-1.9.2 renders the html very weirdly.

The only downside of HTML output is that it's much bigger in size (2 pages PDF = 77 kb, 2 pages in HTML ~= 11 MB). It needs to be gzipped to be considered a candidate: (~900 kb, still 10 times the size).