allow selecting the backend by setting the environment variable MPLBACKEND

This commit adds a new way of selecting the matplotlib backend. The `-d` flag can cause problems with scripts that use the command line arguments as parameters and don't ignore unknown ones. An environment variable `MPL_BACKEND` can be set by the user (or in the code) and is far less likely to cause problems than using the `-d` flag.
@WeatherGod How about this:

    There are a four ways to configure your backend. If they conflict each other,
    the method mentioned last in the following list will be used, e.g. calling
    :func:`~matplotlib.use()` will override the settings in your ``matplotlibrc``.