Allow custom configuration for cached connection

[patricksanders]

This PR adds a config keyword argument to boto3_cached_conn() that allows end users to specify a botocore.client.Config to be passed to the boto3 client creation. This enables a workaround for boto/botocore#2109 by setting the config argument in the sts_conn() decorator:

@sts_conn('s3', config=Config(signature_version='s3v4'))

[coveralls]

Coverage increased (+0.05%) to 34.614% when pulling ee1033d on s3-sigv4 into 428d96f on master.