add filtering when listing containers and images

This includes the **filters** param for **/containers/json** and **/images/json** endpoints.. Used by the **--filter=[]** param of Docker client.
I'd rather extend queryString to also marshall maps, instead of using a pointer to a map, which is not very idiomatic. What do you think?

Does Docker support multiple values for a filter? What's the behavior in this case? I mean, for this code:

```
filters := map[string][]string{"status": {"running", "restarting"}}
```

What does Docker do?