Add env & expandenv template functions

[balboah]

Using environment variables is a common practice when deploying
docker containers. This allows for using environment variables in notifications.

This can also enable functionality described in #504 after adding parsing to the Secret types.

[brian-brazil]

As was explained in #504, we do not allow configuration from the environment. If you want this it is recommended that you use your configuration management system to do so.

This would also go against our security model.