Added serving the output directory if html-based format are selected.

OK, the rational: reveal, and the other html-based formats were born to be viewed in the browser, so I added a new `serve` function inside the FilesWriter class and I am calling it from Nbconvertapp only when the "reveal", "basic_html" or "full_html" are requested.
In this way, the user have immediately access to the served `localhost:8000` and can see their reveal slideshow (or the other html-based files) in one step.

Cheers.

Damián.