fixed pongo2 replaceFilter

pkg/filter/init.go

@@ -8,7 +8,10 @@ import (
 func init() {
 	err := pongo2.ReplaceFilter("markdown", MarkdownFilter)
 	if err != nil {
-		panic(err)
+		err = pongo2.RegisterFilter("markdown", MarkdownFilter)
+		if err != nil {
+			panic(err)
+		}
 	}
 
 	newFilters := map[string]pongo2.FilterFunction{