Add version to CLI usage

[orellazri]

Both as a flag and in the help text.
This is probably natively supported in urfave/cli

[aviadhaham]

@orellazri
seems like the version itself, including the -v / --version flag comes built in with urfave/cli and it's as easy as just adding Version to the App:

	app := &cli.App{
		Name:    "renderkit",
		...
		Version: "0.2.0",
		...
	}

$ go run main.go -v                                          
renderkit version 0.2.0

As for automating the version bump, seems like gomplate does this using his own logic written in go as well
https://github.com/hairyhenderson/gomplate/blob/e3b86e89fca0aad9f5a4f9856f0b57d9cc693470/version/gen/vgen.go

however, I did find this tool that seems interesting: https://github.com/restechnica/semverbot