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.

**Update**
I changed the implementation... now you have a `--serve` flag to call from command line, in this way you are not forced to serve the build directory, it is an option.
To achieve this, I subclass the FileWriter class to add a serve function (not implemented in the FIleWriter class to avoid the serve of the build directory when the flag is not present.

**Update**
Now `--post serve` call a serve post processor.

Thanks.