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/main.go

@@ -0,0 +1,20 @@
+package main
+
+import "github.com/kataras/iris/v12"
+
+func main() {
+	app := iris.New()
+
+	// Read about its markup syntax at: https://github.com/eknkc/amber
+	tmpl := iris.Amber("./views", ".amber")
+
+	app.RegisterView(tmpl)
+
+	app.Get("/", func(ctx iris.Context) {
+		ctx.View("index.amber", iris.Map{
+			"Title": "Title of The Page",
+		})
+	})
+
+	app.Listen(":8080")
+}