Make optional the before argument of enter().insert('')

From the documentation: "For enter selections, the before selector may
be omitted, [...]"

d3/d3.d.ts

@@ -811,7 +811,7 @@ declare module D3 {
 
     export interface EnterSelection {
         append: (name: string) => Selection;
-        insert: (name: string, before: string) => Selection;
+        insert: (name: string, before?: string) => Selection;
         select: (selector: string) => Selection;
         empty: () => boolean;
         node: () => Element;