cli: add Command.setup() (#7850)

vlib/cli/README.md

@@ -0,0 +1,30 @@
+Usage example:
+
+```v
+module main
+
+import os
+import cli
+
+fn main() {
+	mut app := cli.Command{
+		name: 'example-app'
+		description: 'example-app'
+		execute: fn (cmd cli.Command) ? {
+			println('hello app')
+			return
+		}
+		commands: [
+			cli.Command{
+				name: 'sub'
+				execute: fn (cmd cli.Command) ? {
+					println('hello subcommand')
+					return
+				}
+			},
+		]
+	}
+	app.setup()
+	app.parse(os.args)
+}
+```