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/paragraph.go

@@ -0,0 +1,46 @@
+package org
+
+import (
+	"regexp"
+	"strings"
+)
+
+type Paragraph struct{ Children []Node }
+type HorizontalRule struct{}
+
+var horizontalRuleRegexp = regexp.MustCompile(`^(\s*)-{5,}\s*$`)
+var plainTextRegexp = regexp.MustCompile(`^(\s*)(.*)`)
+
+func lexText(line string) (token, bool) {
+	if m := plainTextRegexp.FindStringSubmatch(line); m != nil {
+		return token{"text", len(m[1]), m[2], m}, true
+	}
+	return nilToken, false
+}
+
+func lexHorizontalRule(line string) (token, bool) {
+	if m := horizontalRuleRegexp.FindStringSubmatch(line); m != nil {
+		return token{"horizontalRule", len(m[1]), "", m}, true
+	}
+	return nilToken, false
+}
+
+func (d *Document) parseParagraph(i int, parentStop stopFn) (int, Node) {
+	lines, start := []string{d.tokens[i].content}, i
+	i++
+	stop := func(d *Document, i int) bool {
+		return parentStop(d, i) || d.tokens[i].kind != "text" || d.tokens[i].content == ""
+	}
+	for ; !stop(d, i); i++ {
+		lines = append(lines, d.tokens[i].content)
+	}
+	consumed := i - start
+	return consumed, Paragraph{d.parseInline(strings.Join(lines, "\n"))}
+}
+
+func (d *Document) parseHorizontalRule(i int, parentStop stopFn) (int, Node) {
+	return 1, HorizontalRule{}
+}
+
+func (n Paragraph) String() string      { return orgWriter.nodesAsString(n) }
+func (n HorizontalRule) String() string { return orgWriter.nodesAsString(n) }