Update blackfriday dependency per #2994 (#3857)

Signed-off-by: PJ Eby <pje@telecommunity.com>

vendor/github.com/russross/blackfriday/markdown.go

@@ -13,9 +13,6 @@
 //
 //
 
-// Blackfriday markdown processor.
-//
-// Translates plain text with simple formatting rules into HTML or LaTeX.
 package blackfriday
 
 import (
@@ -46,6 +43,7 @@ const (
 	EXTENSION_AUTO_HEADER_IDS                        // Create the header ID from the text
 	EXTENSION_BACKSLASH_LINE_BREAK                   // translate trailing backslashes into line breaks
 	EXTENSION_DEFINITION_LISTS                       // render definition lists
+	EXTENSION_JOIN_LINES                             // delete newline and join lines
 
 	commonHtmlFlags = 0 |
 		HTML_USE_XHTML |
@@ -161,7 +159,7 @@ var blockTags = map[string]struct{}{
 // Currently Html and Latex implementations are provided
 type Renderer interface {
 	// block-level callbacks
-	BlockCode(out *bytes.Buffer, text []byte, lang string)
+	BlockCode(out *bytes.Buffer, text []byte, infoString string)
 	BlockQuote(out *bytes.Buffer, text []byte)
 	BlockHtml(out *bytes.Buffer, text []byte)
 	Header(out *bytes.Buffer, text func() bool, level int, id string)
@@ -220,7 +218,8 @@ type parser struct {
 	// Footnotes need to be ordered as well as available to quickly check for
 	// presence. If a ref is also a footnote, it's stored both in refs and here
 	// in notes. Slice is nil if footnotes not enabled.
-	notes []*reference
+	notes       []*reference
+	notesRecord map[string]struct{}
 }
 
 func (p *parser) getRef(refid string) (ref *reference, found bool) {
@@ -243,6 +242,11 @@ func (p *parser) getRef(refid string) (ref *reference, found bool) {
 	return ref, found
 }
 
+func (p *parser) isFootnote(ref *reference) bool {
+	_, ok := p.notesRecord[string(ref.link)]
+	return ok
+}
+
 //
 //
 // Public interface
@@ -378,6 +382,7 @@ func MarkdownOptions(input []byte, renderer Renderer, opts Options) []byte {
 
 	if extensions&EXTENSION_FOOTNOTES != 0 {
 		p.notes = make([]*reference, 0)
+		p.notesRecord = make(map[string]struct{})
 	}
 
 	first := firstPass(p, input)
@@ -799,7 +804,17 @@ func ispunct(c byte) bool {
 
 // Test if a character is a whitespace character.
 func isspace(c byte) bool {
-	return c == ' ' || c == '\t' || c == '\n' || c == '\r' || c == '\f' || c == '\v'
+	return ishorizontalspace(c) || isverticalspace(c)
+}
+
+// Test if a character is a horizontal whitespace character.
+func ishorizontalspace(c byte) bool {
+	return c == ' ' || c == '\t'
+}
+
+// Test if a character is a vertical whitespace character.
+func isverticalspace(c byte) bool {
+	return c == '\n' || c == '\r' || c == '\f' || c == '\v'
 }
 
 // Test if a character is letter.