Set 'html5' as default backend

[abelsromero]

Current code and README states that the default backend is docbook.
To avoid confusions adn to align it with what Asciidoctor uses (http://asciidoctor.org/docs/user-manual/#selecting-an-output-format), maybe we should change it to html5.

[mojavelinux]

I'd be fine with that, but probably for 1.6.0. The default docbook was chosen at the time when the primary goal of the plugin was to connect it to the DocBook toolchain, but we've graduated from there :)