Aggregating multiple groups

Any plans to add aggregation of multiple groups? We have a bunch of services running in ECS and it's sometimes useful to be able to view the aggregated logs of multiple of these services at once

The issue names no files, tests, or entry points. Start by tracing how awslogs selects services and retrieves CloudWatch logs, then determine how multiple ECS services should be specified and combined. Done means users can view aggregated logs from multiple services at once.

Written by the indexing model from the issue text.