Usage tag: Keep tag convention

[copitux]

[conf.json]

{
    "foo_bar": "foo",
    "foo_nested": {
        "foo_bar": "fopo"
    }
}

[main.go]

type Config struct {
    FooBar string `usage:"foo" json:"foo_bar"`
    FooNested Nested `usage:"nested" json:"foo_nested"`
}

type Nested struct {
    FooBar string `usage:"foo" json:"foo_bar"`
}

func main() {
    c := &Config{}
    goconfig.Read(c)
    log.Printf("%#v\n",c)
}

$ go run main.go -h
  -config string
    	Configuration JSON file
  -foobar string
    	foo
  -foonested.foobar string
    	foo

$ go run main.go -config conf.json 
 &main.Config{FooBar:"foo", FooNested:main.Nested{FooBar:"foo"}}

[fulldump]

retest this please