Add configurations related to matplotlib's plot directive to the state.

def _insert_matplotlib_configs(c):
    """Add configurations related to matplotlib's plot directive to the state.
    """
    if 'extensions' not in c:
        c['extensions'] = []

    try:
        import matplotlib.sphinxext.plot_directive
        c['extensions'] += [matplotlib.sphinxext.plot_directive.__name__]
    except (ImportError, AttributeError):
        # AttributeError is checked here in case matplotlib is installed but
        # Sphinx isn't.  Note that this module is imported by the config file
        # generator, even if we're not building the docs.
        warnings.warn(
            "matplotlib's plot_directive could not be imported. "
            "Inline plots will not be included in the output.")

    return c