Preserve HTML Text for link render hooks

The context now have two text methods:

* Text - rendered
* PlainText

Fixes #6629

markup/converter/hooks/hooks.go

@@ -24,6 +24,7 @@ type LinkContext interface {
 	Destination() string
 	Title() string
 	Text() string
+	PlainText() string
 }
 
 type Render struct {