Add multi path support

[gsoyka]

• Adds ability to specify multiple colon separated paths, like in the $PATH variable.

[orfin]

Thanks @gsoyka for the contribution.
IMHO we should think how will it work for users that use colon notation for naming parameters.
This may break backward compatibility for use-cases such as: AWS_ENV_PATH=prod:service-1

What do you think? Are we able to handle multiple param names using different notation, or maybe should we move AWS_ENV_PATH as CLI option so it will be more natural to support multiple parameters? $ aws-env --path=prod/foo --path=prod/bar

[chizou]

@orfin We decided to go with using the colon because
a) It mimics every major OS' PATH scheme of colon separated paths
b) AWS Parameter Store doesn't allow colons in the names of paths: https://docs.aws.amazon.com/systems-manager/latest/userguide/sysman-parameter-name-constraints.html

So there would be no conflicts

[ghost]

Any update with this PR? it would be a nice feature