[feat] User defined colors

[hjkatz]

Here: https://github.com/jorgebastida/awslogs/blob/master/awslogs/core.py#L182-L197

It would be lovely if I could specify --group-color and --stream-color explicitly.

Why? I found this script: https://gist.github.com/shollingsworth/3ca630eca73cc55d4461ed8e88e2decf

(edited that script for awslogs locally)

This works nicely, but all the logs are the same color because each invocation is an individual exec of awslogs.