Add ability to modify the global tags 

I would like to be able to make new clients from an existing one, which extended global tags.

```
client := statsd.NewClient(statsd.DefaultTags(statsd.StringTag("foo", "bar")))

extra := []statsd.Tags{statsd.StringTag("url", "/foo")}
child := client.WithTags(extra...)

child.Incr("counter", 10) // Tags: foo=bar,url=/foo
```

let me know what you are your thoughts.