Add instructions for running runipy from Python

This is just an idea in case you are interested - I need to run many notebooks automatically, and am doing so from Python, so I decided to try and use it programmatically. This adds instructions to the README.rst for how to do this and also makes a minor change so that ``pylab`` doesn't have to be given in ``__init__`` if ``False`` (which is default also for the command-line).

The only issue with this is that the logger doesn't get set up this way, but there is probably a solution to this?