Default value incorrect when printing usage

[chrjen]

Version: 0.2.7

The value Hello, %s! as a default value seems to be treated as a format string when it really should
just be printed as is.

Example code:

package main

import (
	"fmt"
	"os"

	"github.com/mkideal/cli"
)

type argT struct {
	Format string `cli:"f,format" dft:"Hello, %s!" usage:"format for greeting"`
}

func main() {
	os.Exit(cli.Run(new(argT), func(ctx *cli.Context) error {
		argv := ctx.Argv().(*argT)
		fmt.Println(argv.Format)
		fmt.Println("==================")
		ctx.WriteUsage()
		return nil
	}))
}

Expected output:

Hello, %s!
==================
Options:

  -f, --format[=Hello, %s!]   format for greeting

Actual output:

Hello, %s!
==================
Options:

  -f, --format[=Hello, %!s(MISSING)!]   format for greeting

It prints out Hello, %!s(MISSING)! instead.