Feature/html5 theme support

@scottnix @emhr Headsup: Let's hide the checkbox for xhtml mode in the theme options by default. Upgrading child themes will need to use `add_theme_support('thematic_html5')` to get the html5 markup. Checkbox is confusing UI to begin with, and this ensures no accidental switches with potential disastrous results.

I added unit tests and tried with upgrading a couple of themes, but more testing is always better.

See #111