Added support of using custom S3 endpoint URL (chimpler#4)

Author: oyarushe

README.md

@@ -193,3 +193,7 @@ psql> select * from animals;
 
 If the file has the metadata `Content-Encoding=gzip` in S3, then the file will be automatically unzipped prior to be copied to the table.
 One can update the metadata in S3 by following the instructions described [here](https://docs.aws.amazon.com/AmazonS3/latest/user-guide/add-object-metadata.html).
+
+## Support custom S3 endpoint URL
+
+If you need to change S3 endpoint URL set environment variable `S3_ENDPOINT_URL`, for example, to use local S3 server for development.

aws_s3--0.0.1.sql

@@ -63,6 +63,8 @@ AS $$
  module_cache[module_name] = _module
  return _module
 
+ os = cache_import('os')
+
  boto3 = cache_import('boto3')
  tempfile = cache_import('tempfile')
  gzip = cache_import('gzip')
@@ -75,7 +77,8 @@ AS $$
  aws_access_key_id=access_key,
  aws_secret_access_key=secret_key,
  aws_session_token=session_token,
- region_name=region
+ region_name=region,
+ endpoint_url=os.environ.get('S3_ENDPOINT_URL')
  )
 
  response = s3.head_object(Bucket=bucket, Key=file_path)