Support URI string parameter for all WebSocket backends

[jordaneremieff]

Only the postgresql backend currently supports passing a single stringuri connection parameter, but it'd be a lot easier to use and customize the parameters if all the backends used a uri parameter.

SQLite, PostgreSQL, and Redis will be easy enough since the libs I use already accept them:

sqlite://<file_path>
postgresql://
redis://
rediss://

The backends that rely on Boto3 requests (S3, dynamodb) will need to be parsed first, but I figure something like this wouldn't be too difficult to implement:

s3://<bucket-name>/<key>?region=<region>
dynamodb://<table_name>?region=<region>&endpoint_url=<endpoint-url>

On a side-note: I should also document the behaviour of S3/dynamodb calls when inside a VPC.

[jordaneremieff]

Closed by #103