feat:use flags by struct

[FengGeSe]

Add two methods to FlagSet：
AddFlags(obj interface{}) error // add flags to FlagSet by struct
SetValues(obj interface{}) error // set struct values from FlagSet

How to use?

type testOptions struct {
    String string `flag:"string" short:"a" default:"abc" desc:"this is string"`
}
flagSet.AddFlags(testOptions{})

var opts = testOptions{}
flagSet.SetValues(&opts)

The two methods support most types eg. int uint bool string float net.IP time.Duration, It also can support nested struct.