Add gen-config command

[themalkolm]

It would be nice to automatically generate config structure for any command e.g.

var RootCmd = &cobra.Command{
	Use:          "foobar",
...
func init() {
	//
	// You need to define flags as usual.
	//
	RootCmd.PersistentFlags().String("foo", "", "Some foonees must be set")
	RootCmd.PersistentFlags().String("foo-bar", "", "Some barness must be set")
...

$ foobar gen-config -o ./config.go
...

$ cat ./config.o
type Config struct {
	Foo    string `mapstructure:"foo"`
	FooBar string `mapstructure:"foo-bar"`
}

So kind of self-updating code. The tricky part is to how to start this process. This could be done simply by having a dummy struct in the beginning to make sure code compiles.

[themalkolm]

What if we generate all flags from a config struct instead?