Add support for streaming all users contained in a twitter public list

`user_lists` is a shortcut to follow all members of a public
twitter list identified by the user id and the list slug (last part of uri when open a list in your browser). The
configuration is the same as the twitter API (a single comma separated string value, or using json arrays).

```
PUT _river/my_twitter_river/_meta
{
    "type" : "twitter",
    "twitter" : {
        "filter" : {
            "tracks" : "test,something,please",
            "follow" : "111,222,333",
            "user_lists" : "ownerScreenName1/slug1,ownerScreenName2/slug2",
            "locations" : "-122.75,36.8,-121.75,37.8,-74,40,-73,41",
            "language" : "fr,en"
        }
    }
}
```

Closes #71.
Internal: clean code

Instead of having inner if conditions, it's better to check that when we want to filter at least one filter has been set.
If not, we exit.

Otherwise, we can apply language filtering if any.