Better markdown docs generation

[tarampampam]

Hi there, and thanks for your project! He is really awesome!

I could ask what you think about the improvement of markdown CLI docs generation - very often I use the following, hm... "addon" (which was crafted by me) to automatically update the CLI usage section in the project readme file: gh.tarampamp.am/urfave-cli-docs. In a few words, it allows using go generate ./... to update a section in the readme file (between the tags):

<!--GENERATED:CLI_DOCS-->
<!--/GENERATED:CLI_DOCS-->

With something like that:

From the sources:

import (
	"fmt"

	"github.com/urfave/cli/v2"
)

func NewApp() *cli.App {
	return &cli.App{
		Name:  "example",
		Usage: "Example CLI application",
		Commands: []*cli.Command{
			{
				Name:    "hello",
				Aliases: []string{"hi"},
				Usage:   "Prints hello",
				Action: func(c *cli.Context) error {
					fmt.Printf("Hello, %s!", c.String("name"))
					return nil
				},
				Flags: []cli.Flag{
					&cli.StringFlag{
						Name:    "name",
						Value:   "stranger",
						Usage:   "Just a name",
						Aliases: []string{"n"},
						EnvVars: []string{"NAME"},
					},
				},
			},
		},
	}
}

And I found this very useful, and probably other users of this library will find this useful and for them. Should I make PR with appropriate changes to this project, or not?

[dearchap]

@tarampampam Please submit a PR . Thank you so much

[tarampampam]

@dearchap @urfave Could anyone review the changes in #1722?