vendor the dependencies

There is a discrepancy of behavior of `github.com/urfave/cli` between
using go1.12 and go1.15, when the dependency is not present as vendored
source. Now this builds fine with go1.12

There are users of tar-split as a package. It is the hope that by adding
this vendored source it does not impact them depending on tar-split
itself.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

vendor/github.com/russross/blackfriday/v2/esc.go

@@ -0,0 +1,34 @@
+package blackfriday
+
+import (
+	"html"
+	"io"
+)
+
+var htmlEscaper = [256][]byte{
+	'&': []byte("&"),
+	'<': []byte("&lt;"),
+	'>': []byte("&gt;"),
+	'"': []byte("&quot;"),
+}
+
+func escapeHTML(w io.Writer, s []byte) {
+	var start, end int
+	for end < len(s) {
+		escSeq := htmlEscaper[s[end]]
+		if escSeq != nil {
+			w.Write(s[start:end])
+			w.Write(escSeq)
+			start = end + 1
+		}
+		end++
+	}
+	if start < len(s) && end <= len(s) {
+		w.Write(s[start:end])
+	}
+}
+
+func escLink(w io.Writer, text []byte) {
+	unesc := html.UnescapeString(string(text))
+	escapeHTML(w, []byte(unesc))
+}