hugolib: Handle shortcode per output format

This commit allows shortcode per output format, a typical use case would be the special AMP media tags.

Note that this will only re-render the "overridden" shortcodes and only  in pages where these are used, so performance in the normal case should not suffer.

Closes #3220

tpl/template.go

@@ -58,6 +58,11 @@ type TemplateExecutor interface {
 	Tree() string
 }
 
+// TemplateDebugger prints some debug info to stdoud.
+type TemplateDebugger interface {
+	Debug()
+}
+
 // TemplateAdapter implements the TemplateExecutor interface.
 type TemplateAdapter struct {
 	Template