Reduce amount of logging messages

[danijar]

Instead of writing a message every time saved, there should be one message in the beginning that states the location of the handout.

[danijar]

This is actually useful, e.g. because text messages should show up in the terminal. But it's easy to disable with

import handout
import logging
logging.getLogger('handout').setLevel(logging.ERROR)

[danijar]

As discussed in #11, doc.add_text() should always print so you can use it to replace print() where needed. Besides this, the path to the index.html could be printed once in the beginning to make it easy to open the handout in the browser. All other messages will be hidden by default (e.g. log level info).