use my fork of blackfriday

Turns out the PR to the upstream is not merged and looks like the
maintainers didn't have a chance to merge it. The `replace` directive
causes some problems: https://github.com/kovetskiy/mark/issues/71

pkg/mark/markdown.go

@@ -7,7 +7,7 @@ import (
 
 	"github.com/kovetskiy/mark/pkg/mark/stdlib"
 	"github.com/reconquest/pkg/log"
-	bf "github.com/russross/blackfriday/v2"
+	bf "github.com/kovetskiy/blackfriday/v2"
 )
 
 type ConfluenceRenderer struct {