Support expiration for new log groups

[laptevn]

Log groups in CloudWatch has expiration setting. I didn't find the way to set this setting in this appender (i.e. logs will never expire using the group created by this appender)
Do you have any plans to implement it?
I can help with adding this support :-)

[josefhernandez]

Default value for expiration days is 7 (if not specified)