Support custom endpoint_url

[Jafnee]

The boto3 library allows settings a custom endpoint:

resource = boto3.resource(
    service_name='s3',
    endpoint_url='http://localhost',
)

Supporting this would allow use of bucketstore with other s3 like services, e.g. localstack, Alibaba Cloud Object Storage Service (OSS)

[fbkopp]

Please lock at #25 feedback would be grateful