No-HTML panel

Most times we can handle a wrapper div around the panel content just fine, but there are some specific cases where that's not true. I've come across a case where wrapper would make some Bootstrap style selector not match my markup, so I decided to create a option on panels called "html", that would be expected to be ```true``` by default, but if set to ```false``` would cause for no wrapper being generated around the panel's content.

Although I'm already using this in a project of mine, I think there is much room for improvement here.