Add reader with support for secret files

[Radiergummi]

On some platforms such as Kubernetes or Docker Swarm, it's possible to pass sensitive configuration values to containers in files instead of environment variables. This is a good thing, and makes applications more secure.
Using secret files usually means the following dance:

1. Check for the variable we need in the process environment (so we can override a secret locally, for example).
2. If it isn't defined, check if a matching secret file exists.
3. If it does, read it, trim the content, and return that; a fallback, if given, otherwise.

Some software systems also use variables suffixed with _PATH, so instead of passing a password as an environment variable, you can pass the path of a file that contains it. This alters the flow to:

1. Check for the variable we need in the process environment.
2. If it isn't defined, check if a variable named {$variable}_PATH exists instead.
3. If it doesn't, or does not contain a valid filesystem path, return a fallback, if given.
4. Read the given path, trim the content, and return it.

It would be cool to have a reader available that does those things! In the larger picture, I'm trying to get this implemented in Laravel (see the related feature request).
Happy to provide a PR.