Require Inlines to provide a best plaintext rendering

This allows markdown to be parsed "inside" the alt
attribute of an image, and then the best plaintext
can be used as the rest.
This improves CommonMark compliance.

src/Components/Inlines/PlainText.php

@@ -72,4 +72,12 @@ final class PlainText implements Inline
             }
         );
     }
+
+    /**
+     * @return Text
+     */
+    public function bestPlaintext()
+    {
+        return new Text($this->text);
+    }
 }