Rewrite markdown rendering to blackfriday v2 and rewrite orgmode rendering to go-org (#8560)

* Rewrite markdown rendering to blackfriday v2.0

* Fix style

* Fix go mod with golang 1.13

* Fix blackfriday v2 import

* Inital orgmode renderer migration to go-org

* Vendor go-org dependency

* Ignore errors :/

* Update go-org to latest version

* Update test

* Fix go-org test

* Remove unneeded code

* Fix comments

* Fix markdown test

* Fix blackfriday regression rendering HTML block

vendor/github.com/niklasfasching/go-org/org/fuzz.go

@@ -0,0 +1,27 @@
+// +build gofuzz
+
+package org
+
+import (
+	"bytes"
+	"strings"
+)
+
+// Fuzz function to be used by https://github.com/dvyukov/go-fuzz
+func Fuzz(input []byte) int {
+	conf := New().Silent()
+	d := conf.Parse(bytes.NewReader(input), "")
+	orgOutput, err := d.Write(NewOrgWriter())
+	if err != nil {
+		panic(err)
+	}
+	htmlOutputA, err := d.Write(NewHTMLWriter())
+	if err != nil {
+		panic(err)
+	}
+	htmlOutputB, err := conf.Parse(strings.NewReader(orgOutput), "").Write(NewHTMLWriter())
+	if htmlOutputA != htmlOutputB {
+		panic("rendered org results in different html than original input")
+	}
+	return 0
+}