Add support for aes-256-gcm cipher #4
Description
Since aes-256-gcm is a stronger cipher, it would be good to have support out of the box, and even make it the default.
The default of aes-128-gcm was chosen as this library was initially designed to work with rails credentials, where aes-256-gcm is the default for credential files.
It's worth noting that rails default for active support encryption of strings is aes-256-gcm internally.