Allow renderers to deal with Line2D objects that have lines and markers.

For renderers wanting to draw the SVG renderings for lines and markers,
it's best to have separate commands, `draw_line`, and `draw_markers`.
However, some high-level renderers will want to know when they can
aggregate these lines and markers into a reference to the same Line2D
object. That is, they don't want import TWO things that represent the
same data.

The base class now accepts an option to `draw_marked_line` that by
default calls both `draw_line` AND `draw_markers`. However, this can be
reimplemented if desired.