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/footnote.go

@@ -0,0 +1,35 @@
+package org
+
+import (
+	"regexp"
+)
+
+type FootnoteDefinition struct {
+	Name     string
+	Children []Node
+	Inline   bool
+}
+
+var footnoteDefinitionRegexp = regexp.MustCompile(`^\[fn:([\w-]+)\](\s+(.+)|\s*$)`)
+
+func lexFootnoteDefinition(line string) (token, bool) {
+	if m := footnoteDefinitionRegexp.FindStringSubmatch(line); m != nil {
+		return token{"footnoteDefinition", 0, m[1], m}, true
+	}
+	return nilToken, false
+}
+
+func (d *Document) parseFootnoteDefinition(i int, parentStop stopFn) (int, Node) {
+	start, name := i, d.tokens[i].content
+	d.tokens[i] = tokenize(d.tokens[i].matches[2])
+	stop := func(d *Document, i int) bool {
+		return parentStop(d, i) ||
+			(isSecondBlankLine(d, i) && i > start+1) ||
+			d.tokens[i].kind == "headline" || d.tokens[i].kind == "footnoteDefinition"
+	}
+	consumed, nodes := d.parseMany(i, stop)
+	definition := FootnoteDefinition{name, nodes, false}
+	return consumed, definition
+}
+
+func (n FootnoteDefinition) String() string { return orgWriter.nodesAsString(n) }