fix(extends): Support multiple extends

Fixes #60

Tests/StencilTests/InheritenceSpec.swift

@@ -13,5 +13,11 @@ func testInheritence() {
       let template = loader.loadTemplate("child.html")
       try expect(try template?.render(context)) == "Header\nChild"
     }
+
+    $0.it("can inherit from another template inheriting from another template") {
+      let context = Context(dictionary: ["loader": loader])
+      let template = loader.loadTemplate("child-child.html")
+      try expect(try template?.render(context)) == "Child Child Header\nChild"
+    }
   }
 }

Tests/StencilTests/fixtures/child-child.html

@@ -0,0 +1,2 @@
+{% extends "child.html" %}
+{% block header %}Child Child Header{% endblock %}