Add ellipse method

Also experimenting with using stroke_preserve, which in turn may make the other methods less repetitive.

Closes #66

samples/draw_shapes.rb

@@ -1,6 +1,7 @@
 require 'squib'
 
 Squib::Deck.new do
+  background color: :white
   rect x: 300, y: 300, width: 400, height: 400,
        fill_color: :blue, stroke_color: :red, stroke_width: 50.0
 
@@ -20,5 +21,9 @@ Squib::Deck.new do
         stroke_width: 12.0, stroke_color: :cyan,
         fill_color: :burgundy
 
+  ellipse x: 50, y: 925, width: 200, height: 100,
+          stroke_width: 5.0, stroke_color: :cyan,
+          fill_color: :burgundy
+
   save_png prefix: 'shape_'
 end