Replace strings in Dancer2 config with environment variables

[mikkoi]

Ref #1628

What this PR changes?

1. After reading the whole config from different files, it traverses the whole tree and if it sees a string with a substring like "${ENV:SOME_ENV}", it changes it to the corresponding environment variable, if that environment variable is defined.
2. Before doing 1), it checks if the config item "env_var_replace" is defined and is boolean true. If so, then it executes 1). Otherwise it skips. This is to maintain backwards compatibility.
3. Unit tests included.

I am using this solution in my build and web application.

[xsawyerx]

I see now that this was closed. I commented on the original ticket with a way that I think we can do this in core.