HTTP Proxy Authentication

Does s3cmd support HTTP proxy authentication? Our environment requires users to authorize with ID and password.

```
$ s3cmd ls s3://foobar/
ERROR: Error parsing xml: mismatched tag: line 65, column 2
ERROR: b'TRIMMED! GATEWAY RESPONSE'
ERROR: S3 error: 407 (Proxy Authentication Required)
```

### ~/.s3cfg
```
proxy_host = proxy.foobar.com
proxy_port = 8080
```