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_pug_1/templates/footer.pug

@@ -0,0 +1,2 @@
+#footer
+  p Copyright (c) foobar

_examples/view/template_pug_1/templates/header.pug

@@ -0,0 +1,4 @@
+head
+  title My Site
+  <!-- script(src='/javascripts/jquery.js')
+  script(src='/javascripts/app.js') -->

_examples/view/template_pug_1/templates/index.pug

@@ -1,20 +1,7 @@
 doctype html
-html(lang="en")
-	head
-		meta(charset="utf-8")
-		title Title
-	body
-		p ads
-		ul
-			li The name is {{.Name}}.
-			li The age is {{.Age}}.
-
-		each _,_ in .Emails
-			div An email is {{.}}
-
-		| {{ with .Jobs }}
-			each _,_ in .
-				div
-				 An employer is {{.Employer}}
-				 and the role is {{.Role}}
-		| {{ end }}
+html
+  include header.pug
+  body
+    h1 My Site
+    p {{ bold "Welcome to my super lame site."}}
+    include footer.pug