Added an html validation check by looking at Nokogiri errors

I know you said in #30 you're not interested in validating HTML, but Nokogiri already looks at the markup and [provides errors](http://www.nokogiri.org/tutorials/ensuring_well_formed_markup.html) when parsing, so providing basic validation is quite easy.

This simple checker helped me catch a few mistakes in my site, so I thought I'd try and see if you find it useful now :smile: 
Yeah. If I want to run this check directly, I shouldn't need to know about the `validate_html` option, should I? If I'm calling it directly, the understanding would be that `validate_html` would be `true`. This is useful if I want to validate the HTML in some documents, but not all.