Add config element to load environment variables from a file

[sbaeumlisberger]

Hi, I implemented a config option to load environment variables from a file.

[odin568]

Hi @oleg-nenashev and @sbaeumlisberger
Would love to have that merged for v2 - exactly what I was looking for.
Reason is I want to generate the configuration as I have around 30 env variables to be defined. Having those in an env file would have the benefit to have a static config file and just generate the variable parts agnostic to any general winsw config layout changes.