Support for docker secrets

It would be really cool if we could use docker secrets to set sensitive parameters (db password, mailer password, etc).

The canonical way for this would be to parse the same variables with `_FILE` suffix that point to a file containing the actual value to the variable. Read the contents of that file and set it as the proper variable, omitting the `_FILE` suffix.

As I see there's already an open pull request (#70) that's supposed to add this. If that's not good for whatever reason I'm happy to take a stab at this.