Add direct to html render mode

[razorheadfx]

Add a render mode that directly outputs the markdown as html with stylesheet included in the header.

For example

grup README.md --offline readme.html

That way one could use the awesome wkhtmltopdf cli tool to directly output a rendered pdf file from a .md file, like so:

 grup README.md --offline readme.html && wkhtmltopdf readme.html readme.pdf && rm readme.html