Docs: using river behind a proxy

Closes #67.
Add proxy settings in elasticsearch.yml

As suggested per comment in #67, see https://github.com/elasticsearch/elasticsearch-river-twitter/issues/67#issuecomment-57597428, we can support proxy settings in `elasticsearch.yml` file:

```yaml
river.twitter.proxy.host: "host"
river.twitter.proxy.port: "port"
river.twitter.proxy.user: "proxy_user_if_any"
river.twitter.proxy.password: "proxy_password_if_any"
```

Closes #74.