Possibly allowing better extensibility to html output

Hi,

I make use of pytest-html extensively and find the plugin generally fantastic.

In my usage, I have resorted to some substantial modifications to the output content. Recently I found myself going a bit too far and running regexes on output to alter them.

I think the following surgical changes would go a long way in avoiding such poor practices:

• Wrapping body content in a div with class content
• wrapping header in a div with class summary
• and wrapping the results in a div with class results

The principal motivation of this would be to allow styling the output.

I think the biggest concern would be whether this will break a bunch of CI tests in the universe that have come to rely on the html output.

I apologize if this is a common request that gets closed repeatedly. I searched around in the issues but didn't find anything specifically asking this.

All three of these changes are trivial to implement and I could easily make a PR on short notice.

Start in pytest_html/plugin.py at the referenced results, body, and header output locations around lines 498, 528, and 546. Review the surrounding HTML generation and existing comments before making the three wrapper changes. Done means the output contains results, content, and summary divs with the requested classes while preserving existing behavior.