Feature - allow providing tag when creating backup

[nvtkaszpir]

Right now when you invoke backup then tag is taken from the current timestamp, which may depend on the moment the code was executed.

What would be more useful would be the option to allow providing custom tag (such as in restore). This would allow to use predictable tag names, for example I could set a backup with a daily timestamp, and not need to write extra logic to detect what tags are available on given location to create a restore tag.
If not provided, then fall back to current timestamp, which would make it backward compatible.

We assume that if user provides custom tag then we fail fast in case of trying to overwrite data if the tag already exists.

You can experiment with the docker image by:

docker pull quay.io/kaszpir/cain:latest