Add :rc:`contour.linewidths` to rcParams

The new config option :rc:`contour.linewidth` allows to control the default

linewidth of contours as a float. When set to ``None``, the linewidths fall

back to :rc:`lines.linewidth`. The config value is overidden as usual

by the ``linewidths`` argument passed to `~.axes.Axes.contour` when

it is not set to ``None``.