Full support of the http.FileSystem on all view engines as requested at #1575

Also, the HandleDir accepts both string and http.FileSystem (interface{}) (like the view's fs)

_examples/view/template_amber_0/views/index.amber

@@ -0,0 +1,11 @@
+extends layouts/main.amber
+
+block append meta
+    meta[name="keywords"][content="These are added by the child template"]
+
+block menu
+    li Item 1
+    li Item 2
+
+block content
+    p Content from child template

_examples/view/template_amber_0/views/layouts/main.amber

@@ -0,0 +1,15 @@
+!!! transitional
+html
+    head
+        title #{Title}
+
+        block meta
+            meta[name="description"][value="This is a sample"]
+
+    body
+        header#mainHeader
+            ul
+                block menu
+
+        div#content
+            block content