Switch to Google-Style docstrings

[MartinThoma]

Currently, our docstrings look like this:

    def add_named_destination(self, title: str, pagenum: int) -> None:
        """
        Add a destination to the output.

        :param str title: Title to use
        :param int pagenum: Page number this destination points at.
        """

I propose to use the Google docstring format instead:

    def add_named_destination(self, title: str, pagenum: int) -> None:
        """
        Add a destination to the output.

        Args:
            title (str): Title to use
            pagenum (int): Page number this destination points at.
        """

There are two reasons why I suggest this change:

1. Readability: I think the Google format is easier to read (in plain text, the rendered results are the same)
2. Darglint: I would like to be able to automatically check our docstrings for correctness. Darglint has issues with the way the type is documented (:param str title: does not work, :param title: would work)

We can use both styles at the same time. Thus we don't need one gigantic PR to adjust the style, but we can make it e.g. module-by-module (and thus hopefully avoid merge conflicts).

What do you think?

[MartinThoma]

There is also the numpydoc style. I like it a lot, but in my experience many people think it's too verbose.