Add read function to read the content of a file

[NicolasDorier]

This has been very useful in my fork for the following usecase:

I wanted to use docker-gen with TOR. When you create a hidden service, TOR create a file at a specific location with the TOR onion host that represent your service. I needed to use this onion host as input for generating my nginx configuration file. So my nginx.tmpl needed to read a file.

[aywan]

Two years later i did almost same thing https://github.com/nginx-proxy/docker-gen/pull/399 The feature not popular, but required in some cases

[josegonzalez]

This was implemented here through the include function, so I think this one can be closed.

[buchdag]

Thanks for for the reminder @josegonzalez 👍