Add mixin to render JS and CSS links

Author: Conengmo

folium/elements.py

@@ -0,0 +1,21 @@
+from branca.element import Figure, Element, JavascriptLink, CssLink
+
+
+class JSCSSMixin(Element):
+    """Render links to external Javascript and CSS resources."""
+
+    default_js = []
+    default_css = []
+
+    def render(self, **kwargs):
+        figure = self.get_root()
+        assert isinstance(figure, Figure), ('You cannot render this Element '
+                                            'if it is not in a Figure.')
+
+        for name, url in self.default_js:
+            figure.header.add_child(JavascriptLink(url), name=name)
+
+        for name, url in self.default_css:
+            figure.header.add_child(CssLink(url), name=name)
+
+        super().render(**kwargs)