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: 
I copied this from the favicon code: https://github.com/gjtorikian/html-proofer/blob/c30c18374ca2607455dbc196e9759f7a526297ae/lib/html/proofer/checks/favicon.rb#L12

Do you still want me to remove it here?