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/util.go

@@ -0,0 +1,19 @@
+package org
+
+func isSecondBlankLine(d *Document, i int) bool {
+	if i-1 <= 0 {
+		return false
+	}
+	t1, t2 := d.tokens[i-1], d.tokens[i]
+	if t1.kind == "text" && t2.kind == "text" && t1.content == "" && t2.content == "" {
+		return true
+	}
+	return false
+}
+
+func isImageOrVideoLink(n Node) bool {
+	if l, ok := n.(RegularLink); ok && l.Kind() == "video" || l.Kind() == "image" {
+		return true
+	}
+	return false
+}