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/doc.go

@@ -0,0 +1,18 @@
+// Package blackfriday is a markdown processor.
+//
+// It translates plain text with simple formatting rules into an AST, which can
+// then be further processed to HTML (provided by Blackfriday itself) or other
+// formats (provided by the community).
+//
+// The simplest way to invoke Blackfriday is to call the Run function. It will
+// take a text input and produce a text output in HTML (or other format).
+//
+// A slightly more sophisticated way to use Blackfriday is to create a Markdown
+// processor and to call Parse, which returns a syntax tree for the input
+// document. You can leverage Blackfriday's parsing for content extraction from
+// markdown documents. You can assign a custom renderer and set various options
+// to the Markdown processor.
+//
+// If you're interested in calling Blackfriday from command line, see
+// https://github.com/russross/blackfriday-tool.
+package blackfriday