add inner_radius for piechart fixes #42 does not work for Multi-series pies

pygal/config.py

@@ -173,6 +173,10 @@ class Config(object):
 
     tooltip_border_radius = Key(0, int, "Look", "Tooltip border radius")
 
+    inner_radius = Key(
+        0, float, "Look", "Piechart inner radius (donut), must be <.9")
+
+
     ############ Label ############
     x_labels = Key(
         None, list, "Label",

pygal/graph/pie.py

@@ -59,7 +59,7 @@ class Pie(Graph):
                 big_radius = radius
             else:
                 big_radius = radius * .9
-                small_radius = 0
+                small_radius = radius * self.config.inner_radius
 
             self.svg.slice(
                 serie_node, slice_, big_radius, small_radius,