ENH: Add ability to specify AWS port through AWS_S3_PORT environment variable
When trying to integrate Pandas CSV reading from s3 for local development (in docker) with containers from [LocalStack](https://github.com/atlassian/localstack) or [Minio](https://github.com/minio/minio) we need to be able to define a custom host as well as a port.

[PR #12198](https://github.com/pandas-dev/pandas/pull/12198) introduces the `AWS_S3_HOST` environment variable, I propose adding the `AWS_S3_PORT` one. Something like:

```python
s3_host = os.environ.get('AWS_S3_HOST','s3.amazonaws.com')
s3_port = os.environ.get('AWS_S3_PORT')

try:
    conn = boto.connect_s3(host=s3_host, port=s3_port)
except boto.exception.NoAuthHandlerFound:
     conn = boto.connect_s3(host=s3_host,anon=True, port=s3_port)
```

This would allow to define something like this in the `docker-compose.yml` and use Minio for serving the csv files from a local s3 for development and AWS for production:

```yaml
environment:
  - AWS_ACCESS_KEY_ID=supersecret
  - AWS_SECRET_ACCESS_KEY=supersecret
  - AWS_S3_HOST=s3local
  - AWS_S3_PORT=9000
  - S3_USE_SIGV4=True
```

This is only applicable for pandas 0.18.X and 0.19.X since 0.20.X uses s3f. I would be willing to submit a PR for this.