Body element missing from rendered HTML

Possibly related to #114 I found that I was unable to apply a global style to the output as there was no `<body>` element in the mark up.

Suggest updating the generated HTML to:

```c#
//build the html for rendering
html = $"{htmlStart}{headStart}{string.Concat(header)}{headEnd}<body>{html}<footer>{string.Concat(footer)}</footer></body>{htmlEnd}";
```