Flag help generation doesn't handle []string type

I have a struct with a field which is a `[]string` but when I call the help flag I have a wrong type and a wrong default value:

```golang
type Etcd struct {
    Endpoints []string `config:"etcd-endpoints"`
}

var etcd Etcd
etcd.Endpoints = []string{"127.0.0.1:2379"}
```
The help output:
```shell
$> bin -h
Usage of bin:
  -etcd-endpoints value
    	 (default etcd-endpoints)
```