Add ability to include default email template values

[Gowiem]

When writing the clients for my email-service there are certain ejs template values I would like to include at the service level instead of at the client level. The main example I have right now is the hostname that the emails should link to. For example, a password reset email should link to different hostnames according to environment: localhost for dev, servicename:port for staging, productionhost.com for prod. Across my different environments I want to be able to include these values within the config or as part of a separate API call to the email-service.

Going to take a crack at this now for my project and will post a PR back if this is something you folks would like to roll in.