connect behind a proxy with authentication

Hi, there. I was using kaggle-api behind a proxy with authentication. After I set kaggle configs with correct credentials using:
```bash
kaggle config set -n proxy -v http://username:password@host:port
```
and ran any commands that require http connection, I got errors like:

WARNING Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ProxyError('Cannot connect to proxy.', OSError('Tunnel connection failed: **407 authenticationrequired**',))'

After some browsing of the source code, I think the problem was due to the proxy authentication info not correctly conveyed to the client settings. I managed to get my connection working by modifying rest.py as follows (starting line 102, added `proxy_headers`):
```python
if configuration.proxy:
if '@' in configuration.proxy:
proxy_url = configuration.proxy.split('://')[0] + '://' + configuration.proxy.split('@')[-1]
proxy_headers = urllib3.make_headers(proxy_basic_auth='@'.join(configuration.proxy.split('://', 1)[1].split('@')[:-1]))
else:
proxy_url = configuration.proxy
proxy_headers = None
self.pool_manager = urllib3.ProxyManager(
num_pools=pools_size,
maxsize=maxsize,
cert_reqs=cert_reqs,
ca_certs=ca_certs,
cert_file=configuration.cert_file,
key_file=configuration.key_file,
proxy_url=proxy_url,
proxy_headers=proxy_headers,
**addition_pool_args
)
```
Though I find that in the original code `urllib3.ProxyManager` with `proxy_url=configuration.proxy` can parse my proxy link into:
```python
urllib3.util.url.Url(scheme='http', auth='username:password', host='host', port=port, path=None, query=None, fragment=None)
```
For your reference, the [urllib3 docs](http://urllib3.readthedocs.io/en/latest/reference/#urllib3.poolmanager.ProxyManager) also indicates using `proxy_headers` for proxy authentication.

I noticed files like rest.py are generated by swagger-codegen and am not sure whether my modifications are appropriate. Look forward to your reply!