使用base64Captcha包的NewDriverString方法，指定source参数

package main
import (
  github.com/mojocn/base64Captcha
)

func GetCaptcha() (id string,image string,err error){
  	store := base64Captcha.DefaultMemStore
	driver := base64Captcha.NewDriverString(
		80,
		240,
		0,
		1,
		4,
		"abcdefghijklmnopqrstuvwxyz",
		nil,
		nil,
		nil,
	)
	captcha := base64Captcha.NewCaptcha(driver, store)
	id, image, _, err = captcha.Generate()
	return
}