main: switch to urfave/cli/v2

For now this is a drop in replacement for all the flags curently
implemented

Fixes: #171

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

vendor/github.com/urfave/cli/v2/cli.go

@@ -0,0 +1,23 @@
+// Package cli provides a minimal framework for creating and organizing command line
+// Go applications. cli is designed to be easy to understand and write, the most simple
+// cli application can be written as follows:
+//   func main() {
+//     (&cli.App{}).Run(os.Args)
+//   }
+//
+// Of course this application does not do much, so let's make this an actual application:
+//   func main() {
+//     app := &cli.App{
+//			 Name: "greet",
+//			 Usage: "say a greeting",
+//			 Action: func(c *cli.Context) error {
+//				 fmt.Println("Greetings")
+//				 return nil
+//			 },
+//		 }
+//
+//     app.Run(os.Args)
+//   }
+package cli
+
+//go:generate go run flag-gen/main.go flag-gen/assets_vfsdata.go